Unit testing tips by examples in PHP

Introduction

In these times, the benefits of writing unit tests are huge. I think that most of the recently started projects contain any unit tests. In enterprise applications with a lot of business logic, unit tests are the most important tests, because they are fast and can us instantly assure that our implementation is correct. However, I often see a problem with good tests in projects, though these tests' benefits are only huge when you have good unit tests. So in these examples, I will try to share some tips on what to do to write good unit tests.

Table of Contents

1. Introduction
2. Test doubles
3. Naming
4. AAA pattern
5. Object mother
6. Parameterized test
7. Two schools of unit testing
   • Classical
   • Mockist
   • Dependencies
8. Mock vs Stub
9. Three styles of unit testing
   • Output
   • State
   • Communication
10. Functional architecture and tests
11. Observable behavior vs implementation details
12. Unit of behavior
13. Humble pattern
14. Trivial test
15. Fragile test
16. Test fixtures
17. General testing anti-patterns
    • Exposing private state
    • Leaking domain details
    • Mocking concrete classes
    • Testing private methods
    • Time as a volatile dependency
18. 100% Test Coverage shouldn't be the goal
19. Recommended books

Test doubles

Test doubles are fake dependencies used in tests.

Stubs

Dummy

A dummy is a just simple implementation that does nothing.

final class Mailer implements MailerInterface
{
    public function send(Message $message): void
    {
    }
}

Fake

A fake is a simplified implementation to simulate the original behavior.

final class InMemoryCustomerRepository implements CustomerRepositoryInterface
{
    /**
     * @var Customer[]
     */
    private array $customers;

    public function __construct()
    {
        $this->customers = [];
    }

    public function store(Customer $customer): void
    {
        $this->customers[(string) $customer->id()->id()] = $customer;
    }

    public function get(CustomerId $id): Customer
    {
        if (!isset($this->customers[(string) $id->id()])) {
            throw new CustomerNotFoundException();
        }

        return $this->customers[(string) $id->id()];
    }

    public function findByEmail(Email $email): Customer
    {
        foreach ($this->customers as $customer) {
            if ($customer->getEmail()->isEqual($email)) {
                return $customer;
            }
        }

        throw new CustomerNotFoundException();
    }
}

Stub

A stub is the simplest implementation with a hardcoded behavior.

final class UniqueEmailSpecificationStub implements UniqueEmailSpecificationInterface
{
    public function isUnique(Email $email): bool
    {
        return true;
    }
}

$specificationStub = $this->createStub(UniqueEmailSpecificationInterface::class);
$specificationStub->method('isUnique')->willReturn(true);

Mocks

Spy

A spy is an implementation to verify a specific behavior.

final class Mailer implements MailerInterface
{
    /**
     * @var Message[]
     */
    private array $messages;
    
    public function __construct()
    {
        $this->messages = [];
    }

    public function send(Message $message): void
    {
        $this->messages[] = $message;
    }

    public function getCountOfSentMessages(): int
    {
        return count($this->messages);
    }
}

Mock

A mock is a configured imitation to verify calls on a collaborator.

$message = new Message('[email protected]', 'Test', 'Test test test');
$mailer = $this->createMock(MailerInterface::class);
$mailer
    ->expects($this->once())
    ->method('send')
    ->with($this->equalTo($message));

❗️ To verify incoming interactions, use a stub, but to verify outcoming interactions, use a mock. More: Mock vs Stub

Naming

➖ Not good:

public function test(): void
{
    $subscription = SubscriptionMother::new();

    $subscription->activate();

    self::assertSame(Status::activated(), $subscription->status());
}

✔️ Specify explicitly what you are testing

public function sut(): void
{
    // sut = System under test
    $sut = SubscriptionMother::new();

    $sut->activate();

    self::assertSame(Status::activated(), $sut->status());
}

➖ Not good:

public function it_throws_invalid_credentials_exception_when_sign_in_with_invalid_credentials(): void
{

}

public function testCreatingWithATooShortPasswordIsNotPossible(): void
{

}

public function testDeactivateASubscription(): void
{

}

✔️ Better:

• Using underscore improves readability
• The name should describe the behavior, not the implementation
• Use names without technical keywords. It should be readable for a non-programmer person.

public function sign_in_with_invalid_credentials_is_not_possible(): void
{

}

public function creating_with_a_too_short_password_is_not_possible(): void
{

}

public function deactivating_an_activated_subscription_is_valid(): void
{

}

public function deactivating_an_inactive_subscription_is_invalid(): void
{

}

ℹ️ Describing the behavior is important in testing the domain scenarios. If your code is just a utility one it's less important.

❓ Why would it be useful for a non-programmer to read unit tests?

If there is a project with complex domain logic, this logic must be very clear for everyone, so then tests describe domain details without technical keywords, and you can talk with a business in a language like in these tests.

All code that is related to the domain should be free from technical details. A non-programmer won't be read these tests. If you want to talk about the domain these tests will be useful to know what this domain does. There will be a description without technical details e.g., returns null, throws an exception, etc. This kind of information has nothing to do with the domain, so we shouldn't use these keywords.

AAA pattern

It's also common Given, When, Then.

✔️ Separate three sections of the test:

• Arrange: Bring the system under test in the desired state. Prepare dependencies, arguments and finally construct the SUT.
• Act: Invoke a tested element.
• Assert: Verify the result, the final state, or the communication with collaborators.

public function aaa_pattern_example_test(): void
{
    //Arrange|Given
    $sut = SubscriptionMother::new();

    //Act|When
    $sut->activate();

    //Assert|Then
    self::assertSame(Status::activated(), $sut->status());
}

Object mother

The pattern helps to create specific objects which can be reused in a few tests. Because of that the arrange section is concise and the test as a whole is more readable.

final class SubscriptionMother
{
    public static function new(): Subscription
    {
        return new Subscription();
    }

    public static function activated(): Subscription
    {
        $subscription = new Subscription();
        $subscription->activate();
        return $subscription;
    }

    public static function deactivated(): Subscription
    {
        $subscription = self::activated();
        $subscription->deactivate();
        return $subscription;
    }
}

final class ExampleTest
{
    public function example_test_with_activated_subscription(): void
    {
        $activatedSubscription = SubscriptionMother::activated();

        // do something

        // check something
    }

    public function example_test_with_deactivated_subscription(): void
    {
        $deactivatedSubscription = SubscriptionMother::deactivated();

        // do something

        // check something
    }
}

Parameterized test

The parameterized test is a good option to test the SUT with many parameters without repeating the code.

👎 This kind of test is less readable. To increase the readability a little, negative and positive examples should be split up to different tests.

final class ExampleTest extends TestCase
{
    /**
     * @test
     * @dataProvider getInvalidEmails
     */
    public function detects_an_invalid_email_address(string $email): void
    {
        $sut = new EmailValidator();

        $result = $sut->isValid($email);

        self::assertFalse($result);
    }

    /**
     * @test
     * @dataProvider getValidEmails
     */
    public function detects_an_valid_email_address(string $email): void
    {
        $sut = new EmailValidator();

        $result = $sut->isValid($email);

        self::assertTrue($result);
    }

    public function getInvalidEmails(): array
    {
        return [
            ['test'],
            ['[email protected]'],
            ['[email protected]'],
            //...
        ];
    }

    public function getValidEmails(): array
    {
        return [
            ['[email protected]'],
            ['[email protected]'],
            ['[email protected]'],
            //...
        ];
    }
}

Two schools of unit testing

Classical (Detroit school)

• The unit is a single unit of behavior, it can be a few related classes.
• Every test should be isolated from others. So it must be possible to invoke them in parallel or in any order.

final class TestExample extends TestCase
{
    /**
     * @test
     */
    public function suspending_an_subscription_with_can_always_suspend_policy_is_always_possible(): void
    {
        $canAlwaysSuspendPolicy = new CanAlwaysSuspendPolicy();
        $sut = new Subscription();

        $result = $sut->suspend($canAlwaysSuspendPolicy);

        self::assertTrue($result);
        self::assertSame(Status::suspend(), $sut->status());
    }
}

Mockist (London school)

• The unit is a single class.
• The unit should be isolated from all collaborators.

final class TestExample extends TestCase
{
    /**
     * @test
     */
    public function suspending_an_subscription_with_can_always_suspend_policy_is_always_possible(): void
    {
        $canAlwaysSuspendPolicy = $this->createStub(SuspendingPolicyInterface::class);
        $canAlwaysSuspendPolicy->method('suspend')->willReturn(true);
        $sut = new Subscription();

        $result = $sut->suspend($canAlwaysSuspendPolicy);

        self::assertTrue($result);
        self::assertSame(Status::suspend(), $sut->status());
    }
}

ℹ️ The classical approach is better to avoid fragile tests.

Dependencies

[TODO]

Mock vs. Stub

Example:

final class NotificationService
{
    public function __construct(
        private MailerInterface $mailer,
        private MessageRepositoryInterface $messageRepository
    ) {}

    public function send(): void
    {
        $messages = $this->messageRepository->getAll();
        foreach ($messages as $message) {
            $this->mailer->send($message);
        }
    }
}

❌ Bad:

• Asserting interactions with stubs leads to fragile tests

final class TestExample extends TestCase
{
    /**
     * @test
     */
    public function sends_all_notifications(): void
    {
        $message1 = new Message();
        $message2 = new Message();
        $messageRepository = $this->createMock(MessageRepositoryInterface::class);
        $messageRepository->method('getAll')->willReturn([$message1, $message2]);
        $mailer = $this->createMock(MailerInterface::class);
        $sut = new NotificationService($mailer, $messageRepository);

        $messageRepository->expects(self::once())->method('getAll');
        $mailer->expects(self::exactly(2))->method('send')
            ->withConsecutive([self::equalTo($message1)], [self::equalTo($message2)]);

        $sut->send();
    }
}

✔️ Good:

final class TestExample extends TestCase
{
    /**
     * @test
     */
    public function sends_all_notifications(): void
    {
        $message1 = new Message();
        $message2 = new Message();
        $messageRepository = $this->createStub(MessageRepositoryInterface::class);
        $messageRepository->method('getAll')->willReturn([$message1, $message2]);
        $mailer = $this->createMock(MailerInterface::class);
        $sut = new NotificationService($mailer, $messageRepository);

        // Removed asserting interactions with the stub
        $mailer->expects(self::exactly(2))->method('send')
            ->withConsecutive([self::equalTo($message1)], [self::equalTo($message2)]);

        $sut->send();
    }
}

Three styles of unit testing

Output

✔️ The best option:

• The best resistance to refactoring
• The best accuracy
• The lowest cost of maintainability
• If it is possible, you should prefer this kind of test

final class ExampleTest extends TestCase
{
    /**
     * @test
     * @dataProvider getInvalidEmails
     */
    public function detects_an_invalid_email_address(string $email): void
    {
        $sut = new EmailValidator();

        $result = $sut->isValid($email);

        self::assertFalse($result);
    }

    /**
     * @test
     * @dataProvider getValidEmails
     */
    public function detects_an_valid_email_address(string $email): void
    {
        $sut = new EmailValidator();

        $result = $sut->isValid($email);

        self::assertTrue($result);
    }

    public function getInvalidEmails(): array
    {
        return [
            ['test'],
            ['[email protected]'],
            ['[email protected]'],
            //...
        ];
    }

    public function getValidEmails(): array
    {
        return [
            ['[email protected]'],
            ['[email protected]'],
            ['[email protected]'],
            //...
        ];
    }
}

State

✅ Worse option:

• Worse resistance to refactoring
• Worse accuracy
• Higher cost of maintainability

final class ExampleTest extends TestCase
{
    /**
     * @test
     */
    public function adding_an_item_to_cart(): void
    {
        $item = new CartItem('Product');
        $sut = new Cart();

        $sut->addItem($item);

        self::assertSame(1, $sut->getCount());
        self::assertSame($item, $sut->getItems()[0]);
    }
}

Communication

✅ The worst option:

• The worst resistance to refactoring
• The worst accuracy
• The highest cost of maintainability

final class ExampleTest extends TestCase
{
    /**
     * @test
     */
    public function sends_all_notifications(): void
    {
        $message1 = new Message();
        $message2 = new Message();
        $messageRepository = $this->createStub(MessageRepositoryInterface::class);
        $messageRepository->method('getAll')->willReturn([$message1, $message2]);
        $mailer = $this->createMock(MailerInterface::class);
        $sut = new NotificationService($mailer, $messageRepository);

        $mailer->expects(self::exactly(2))->method('send')
            ->withConsecutive([self::equalTo($message1)], [self::equalTo($message2)]);

        $sut->send();
    }
}

Functional architecture and tests

❌ Bad:

final class NameService
{
    public function __construct(private CacheStorageInterface $cacheStorage) {}

    public function loadAll(): void
    {
        $namesCsv = array_map('str_getcsv', file(__DIR__.'/../names.csv'));
        $names = [];

        foreach ($namesCsv as $nameData) {
            if (!isset($nameData[0], $nameData[1])) {
                continue;
            }

            $names[] = new Name($nameData[0], new Gender($nameData[1]));
        }

        $this->cacheStorage->store('names', $names);
    }
}

How to test a code like this? It is possible only with an integration test because it directly uses an infrastructure code related to a file system.

✔️ Good:

Like in functional architecture, we need to separate a code with side effects and code that contains only logic.

final class NameParser
{
    /**
     * @param array $namesData
     * @return Name[]
     */
    public function parse(array $namesData): array
    {
        $names = [];

        foreach ($namesData as $nameData) {
            if (!isset($nameData[0], $nameData[1])) {
                continue;
            }

            $names[] = new Name($nameData[0], new Gender($nameData[1]));
        }

        return $names;
    }
}

final class CsvNamesFileLoader
{
    public function load(): array
    {
        return array_map('str_getcsv', file(__DIR__.'/../names.csv'));
    }
}

final class ApplicationService
{
    public function __construct(
        private CsvNamesFileLoader $fileLoader,
        private NameParser $parser,
        private CacheStorageInterface $cacheStorage
    ) {}

    public function loadNames(): void
    {
        $namesData = $this->fileLoader->load();
        $names = $this->parser->parse($namesData);
        $this->cacheStorage->store('names', $names);
    }
}

final class ValidUnitExampleTest extends TestCase
{
    /**
     * @test
     */
    public function parse_all_names(): void
    {
        $namesData = [
            ['John', 'M'],
            ['Lennon', 'U'],
            ['Sarah', 'W']
        ];
        $sut = new NameParser();

        $result = $sut->parse($namesData);
        
        self::assertSame(
            [
                new Name('John', new Gender('M')),
                new Name('Lennon', new Gender('U')),
                new Name('Sarah', new Gender('W'))
            ],
            $result
        );
    }
}

Observable behavior vs. implementation details

❌ Bad:

final class ApplicationService
{
    public function __construct(private SubscriptionRepositoryInterface $subscriptionRepository) {}

    public function renewSubscription(int $subscriptionId): bool
    {
        $subscription = $this->subscriptionRepository->findById($subscriptionId);

        if (!$subscription->getStatus()->isEqual(Status::expired())) {
            return false;
        }

        $subscription->setStatus(Status::active());
        $subscription->setModifiedAt(new \DateTimeImmutable());
        return true;
    }
}

final class Subscription
{
    private Status $status;

    private \DateTimeImmutable $modifiedAt;

    public function __construct(Status $status, \DateTimeImmutable $modifiedAt)
    {
        $this->status = $status;
        $this->modifiedAt = $modifiedAt;
    }

    public function getStatus(): Status
    {
        return $this->status;
    }

    public function setStatus(Status $status): void
    {
        $this->status = $status;
    }

    public function getModifiedAt(): \DateTimeImmutable
    {
        return $this->modifiedAt;
    }

    public function setModifiedAt(\DateTimeImmutable $modifiedAt): void
    {
        $this->modifiedAt = $modifiedAt;
    }
}

final class InvalidTestExample extends TestCase
{
    /**
     * @test
     */
    public function renew_an_expired_subscription_is_possible(): void
    {
        $modifiedAt = new \DateTimeImmutable();
        $expiredSubscription = new Subscription(Status::expired(), $modifiedAt);
        $repository = $this->createStub(SubscriptionRepositoryInterface::class);
        $repository->method('findById')->willReturn($expiredSubscription);
        $sut = new ApplicationService($repository);

        $result = $sut->renewSubscription(1);

        self::assertSame(Status::active(), $expiredSubscription->getStatus());
        self::assertGreaterThan($modifiedAt, $expiredSubscription->getModifiedAt());
        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function renew_an_active_subscription_is_not_possible(): void
    {
        $modifiedAt = new \DateTimeImmutable();
        $activeSubscription = new Subscription(Status::active(), $modifiedAt);
        $repository = $this->createStub(SubscriptionRepositoryInterface::class);
        $repository->method('findById')->willReturn($activeSubscription);
        $sut = new ApplicationService($repository);

        $result = $sut->renewSubscription(1);

        self::assertSame($modifiedAt, $activeSubscription->getModifiedAt());
        self::assertFalse($result);
    }
}

✔️ Good:

final class ApplicationService
{
    public function __construct(private SubscriptionRepositoryInterface $subscriptionRepository) {}

    public function renewSubscription(int $subscriptionId): bool
    {
        $subscription = $this->subscriptionRepository->findById($subscriptionId);
        return $subscription->renew(new \DateTimeImmutable());
    }
}

final class Subscription
{
    private Status $status;

    private \DateTimeImmutable $modifiedAt;

    public function __construct(\DateTimeImmutable $modifiedAt)
    {
        $this->status = Status::new();
        $this->modifiedAt = $modifiedAt;
    }

    public function renew(\DateTimeImmutable $modifiedAt): bool
    {
        if (!$this->status->isEqual(Status::expired())) {
            return false;
        }

        $this->status = Status::active();
        $this->modifiedAt = $modifiedAt;
        return true;
    }

    public function active(\DateTimeImmutable $modifiedAt): void
    {
        //simplified
        $this->status = Status::active();
        $this->modifiedAt = $modifiedAt;
    }

    public function expire(\DateTimeImmutable $modifiedAt): void
    {
        //simplified
        $this->status = Status::expired();
        $this->modifiedAt = $modifiedAt;
    }

    public function isActive(): bool
    {
        return $this->status->isEqual(Status::active());
    }
}

final class ValidTestExample extends TestCase
{
    /**
     * @test
     */
    public function renew_an_expired_subscription_is_possible(): void
    {
        $expiredSubscription = SubscriptionMother::expired();
        $repository = $this->createStub(SubscriptionRepositoryInterface::class);
        $repository->method('findById')->willReturn($expiredSubscription);
        $sut = new ApplicationService($repository);

        $result = $sut->renewSubscription(1);

        // skip checking modifiedAt as it's not a part of observable behavior. To check this value we
        // would have to add a getter for modifiedAt, probably only for test purposes.
        self::assertTrue($expiredSubscription->isActive());
        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function renew_an_active_subscription_is_not_possible(): void
    {
        $activeSubscription = SubscriptionMother::active();
        $repository = $this->createStub(SubscriptionRepositoryInterface::class);
        $repository->method('findById')->willReturn($activeSubscription);
        $sut = new ApplicationService($repository);

        $result = $sut->renewSubscription(1);

        self::assertTrue($activeSubscription->isActive());
        self::assertFalse($result);
    }
}

ℹ️ The first subscription model has a bad design. To invoke one business operation you need to call three methods. Also using getters to verify operation is not a good practice. In this case, it's skipped checking a change of modifiedAt, probably setting specific modifiedAt during a renew operation can be tested with an expiration business operation. The getter for modifiedAt is not required. Of course, there are cases where finding the possibility to avoid getters provided only for tests will be very hard, but always we should try not to introduce them.

Unit of behavior

❌ Bad:

class CannotSuspendExpiredSubscriptionPolicy implements SuspendingPolicyInterface
{
    public function suspend(Subscription $subscription, \DateTimeImmutable $at): bool
    {
        if ($subscription->isExpired()) {
            return false;
        }

        return true;
    }
}

class CannotSuspendExpiredSubscriptionPolicyTest extends TestCase
{
    /**
     * @test
     */
    public function it_returns_true_when_a_subscription_is_expired(): void
    {
        $policy = new CannotSuspendExpiredSubscriptionPolicy();
        $subscription = $this->createStub(Subscription::class);
        $subscription->method('isExpired')->willReturn(true);

        self::assertFalse($policy->suspend($subscription, new \DateTimeImmutable()));
    }

    /**
     * @test
     */
    public function it_returns_false_when_a_subscription_is_not_expired(): void
    {
        $policy = new CannotSuspendExpiredSubscriptionPolicy();
        $subscription = $this->createStub(Subscription::class);
        $subscription->method('isExpired')->willReturn(false);

        self::assertTrue($policy->suspend($subscription, new \DateTimeImmutable()));
    }
}

class CannotSuspendNewSubscriptionPolicy implements SuspendingPolicyInterface
{
    public function suspend(Subscription $subscription, \DateTimeImmutable $at): bool
    {
        if ($subscription->isNew()) {
            return false;
        }

        return true;
    }
}

class CannotSuspendNewSubscriptionPolicyTest extends TestCase
{
    /**
     * @test
     */
    public function it_returns_false_when_a_subscription_is_new(): void
    {
        $policy = new CannotSuspendNewSubscriptionPolicy();
        $subscription = $this->createStub(Subscription::class);
        $subscription->method('isNew')->willReturn(true);

        self::assertFalse($policy->suspend($subscription, new \DateTimeImmutable()));
    }

    /**
     * @test
     */
    public function it_returns_true_when_a_subscription_is_not_new(): void
    {
        $policy = new CannotSuspendNewSubscriptionPolicy();
        $subscription = $this->createStub(Subscription::class);
        $subscription->method('isNew')->willReturn(false);

        self::assertTrue($policy->suspend($subscription, new \DateTimeImmutable()));
    }
}

class CanSuspendAfterOneMonthPolicy implements SuspendingPolicyInterface
{
    public function suspend(Subscription $subscription, \DateTimeImmutable $at): bool
    {
        $oneMonthEarlierDate = \DateTime::createFromImmutable($at)->sub(new \DateInterval('P1M'));

        return $subscription->isOlderThan(\DateTimeImmutable::createFromMutable($oneMonthEarlierDate));
    }
}

class CanSuspendAfterOneMonthPolicyTest extends TestCase
{
    /**
     * @test
     */
    public function it_returns_true_when_a_subscription_is_older_than_one_month(): void
    {
        $date = new \DateTimeImmutable('2021-01-29');
        $policy = new CanSuspendAfterOneMonthPolicy();
        $subscription = new Subscription(new \DateTimeImmutable('2020-12-28'));

        self::assertTrue($policy->suspend($subscription, $date));
    }

    /**
     * @test
     */
    public function it_returns_false_when_a_subscription_is_not_older_than_one_month(): void
    {
        $date = new \DateTimeImmutable('2021-01-29');
        $policy = new CanSuspendAfterOneMonthPolicy();
        $subscription = new Subscription(new \DateTimeImmutable('2020-01-01'));

        self::assertTrue($policy->suspend($subscription, $date));
    }
}

class Status
{
    private const EXPIRED = 'expired';
    private const ACTIVE = 'active';
    private const NEW = 'new';
    private const SUSPENDED = 'suspended';

    private string $status;

    private function __construct(string $status)
    {
        $this->status = $status;
    }

    public static function expired(): self
    {
        return new self(self::EXPIRED);
    }

    public static function active(): self
    {
        return new self(self::ACTIVE);
    }

    public static function new(): self
    {
        return new self(self::NEW);
    }

    public static function suspended(): self
    {
        return new self(self::SUSPENDED);
    }

    public function isEqual(self $status): bool
    {
        return $this->status === $status->status;
    }
}

class StatusTest extends TestCase
{
    public function testEquals(): void
    {
        $status1 = Status::active();
        $status2 = Status::active();

        self::assertTrue($status1->isEqual($status2));
    }

    public function testNotEquals(): void
    {
        $status1 = Status::active();
        $status2 = Status::expired();

        self::assertFalse($status1->isEqual($status2));
    }
}

class SubscriptionTest extends TestCase
{
    /**
     * @test
     */
    public function suspending_a_subscription_is_possible_when_a_policy_returns_true(): void
    {
        $policy = $this->createMock(SuspendingPolicyInterface::class);
        $policy->expects($this->once())->method('suspend')->willReturn(true);
        $sut = new Subscription(new \DateTimeImmutable());

        $result = $sut->suspend($policy, new \DateTimeImmutable());

        self::assertTrue($result);
        self::assertTrue($sut->isSuspended());
    }

    /**
     * @test
     */
    public function suspending_a_subscription_is_not_possible_when_a_policy_returns_false(): void
    {
        $policy = $this->createMock(SuspendingPolicyInterface::class);
        $policy->expects($this->once())->method('suspend')->willReturn(false);
        $sut = new Subscription(new \DateTimeImmutable());

        $result = $sut->suspend($policy, new \DateTimeImmutable());

        self::assertFalse($result);
        self::assertFalse($sut->isSuspended());
    }

    /**
     * @test
     */
    public function it_returns_true_when_a_subscription_is_older_than_one_month(): void
    {
        $date = new \DateTimeImmutable();
        $futureDate = $date->add(new \DateInterval('P1M'));
        $sut = new Subscription($date);

        self::assertTrue($sut->isOlderThan($futureDate));
    }

    /**
     * @test
     */
    public function it_returns_false_when_a_subscription_is_not_older_than_one_month(): void
    {
        $date = new \DateTimeImmutable();
        $futureDate = $date->add(new \DateInterval('P1D'));
        $sut = new Subscription($date);

        self::assertTrue($sut->isOlderThan($futureDate));
    }
}

❗️ Do not write code 1:1, 1 class : 1 test. It leads to fragile tests which make that refactoring is tough.

✔️ Good:

final class CannotSuspendExpiredSubscriptionPolicy implements SuspendingPolicyInterface
{
    public function suspend(Subscription $subscription, \DateTimeImmutable $at): bool
    {
        if ($subscription->isExpired()) {
            return false;
        }

        return true;
    }
}

final class CannotSuspendNewSubscriptionPolicy implements SuspendingPolicyInterface
{
    public function suspend(Subscription $subscription, \DateTimeImmutable $at): bool
    {
        if ($subscription->isNew()) {
            return false;
        }

        return true;
    }
}

final class CanSuspendAfterOneMonthPolicy implements SuspendingPolicyInterface
{
    public function suspend(Subscription $subscription, \DateTimeImmutable $at): bool
    {
        $oneMonthEarlierDate = \DateTime::createFromImmutable($at)->sub(new \DateInterval('P1M'));

        return $subscription->isOlderThan(\DateTimeImmutable::createFromMutable($oneMonthEarlierDate));
    }
}

final class Status
{
    private const EXPIRED = 'expired';
    private const ACTIVE = 'active';
    private const NEW = 'new';
    private const SUSPENDED = 'suspended';

    private string $status;

    private function __construct(string $status)
    {
        $this->status = $status;
    }

    public static function expired(): self
    {
        return new self(self::EXPIRED);
    }

    public static function active(): self
    {
        return new self(self::ACTIVE);
    }

    public static function new(): self
    {
        return new self(self::NEW);
    }

    public static function suspended(): self
    {
        return new self(self::SUSPENDED);
    }

    public function isEqual(self $status): bool
    {
        return $this->status === $status->status;
    }
}

final class Subscription
{
    private Status $status;

    private \DateTimeImmutable $createdAt;

    public function __construct(\DateTimeImmutable $createdAt)
    {
        $this->status = Status::new();
        $this->createdAt = $createdAt;
    }

    public function suspend(SuspendingPolicyInterface $suspendingPolicy, \DateTimeImmutable $at): bool
    {
        $result = $suspendingPolicy->suspend($this, $at);
        if ($result) {
            $this->status = Status::suspended();
        }

        return $result;
    }

    public function isOlderThan(\DateTimeImmutable $date): bool
    {
        return $this->createdAt < $date;
    }

    public function activate(): void
    {
        $this->status = Status::active();
    }

    public function expire(): void
    {
        $this->status = Status::expired();
    }

    public function isExpired(): bool
    {
        return $this->status->isEqual(Status::expired());
    }

    public function isActive(): bool
    {
        return $this->status->isEqual(Status::active());
    }

    public function isNew(): bool
    {
        return $this->status->isEqual(Status::new());
    }

    public function isSuspended(): bool
    {
        return $this->status->isEqual(Status::suspended());
    }
}

final class SubscriptionSuspendingTest extends TestCase
{
    /**
     * @test
     */
    public function suspending_an_expired_subscription_with_cannot_suspend_expired_policy_is_not_possible(): void
    {
        $sut = new Subscription(new \DateTimeImmutable());
        $sut->activate();
        $sut->expire();

        $result = $sut->suspend(new CannotSuspendExpiredSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertFalse($result);
    }

    /**
     * @test
     */
    public function suspending_a_new_subscription_with_cannot_suspend_new_policy_is_not_possible(): void
    {
        $sut = new Subscription(new \DateTimeImmutable());

        $result = $sut->suspend(new CannotSuspendNewSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertFalse($result);
    }

    /**
     * @test
     */
    public function suspending_an_active_subscription_with_cannot_suspend_new_policy_is_possible(): void
    {
        $sut = new Subscription(new \DateTimeImmutable());
        $sut->activate();

        $result = $sut->suspend(new CannotSuspendNewSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function suspending_an_active_subscription_with_cannot_suspend_expired_policy_is_possible(): void
    {
        $sut = new Subscription(new \DateTimeImmutable());
        $sut->activate();

        $result = $sut->suspend(new CannotSuspendExpiredSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function suspending_an_subscription_before_a_one_month_is_not_possible(): void
    {
        $sut = new Subscription(new \DateTimeImmutable('2020-01-01'));

        $result = $sut->suspend(new CanSuspendAfterOneMonthPolicy(), new \DateTimeImmutable('2020-01-10'));

        self::assertFalse($result);
    }

    /**
     * @test
     */
    public function suspending_an_subscription_after_a_one_month_is_possible(): void
    {
        $sut = new Subscription(new \DateTimeImmutable('2020-01-01'));

        $result = $sut->suspend(new CanSuspendAfterOneMonthPolicy(), new \DateTimeImmutable('2020-02-02'));

        self::assertTrue($result);
    }
}

Humble pattern

How to properly unit test a class like this?

class ApplicationService
{
    public function __construct(
        private OrderRepository $orderRepository,
        private FormRepository $formRepository
    ) {}

    public function changeFormStatus(int $orderId): void
    {
        $order = $this->orderRepository->getById($orderId);
        $soapResponse = $this->getSoapClient()->getStatusByOrderId($orderId);
        $form = $this->formRepository->getByOrderId($orderId);
        $form->setStatus($soapResponse['status']);
        $form->setModifiedAt(new \DateTimeImmutable());

        if ($soapResponse['status'] === 'accepted') {
            $order->setStatus('paid');
        }

        $this->formRepository->save($form);
        $this->orderRepository->save($order);
    }

    private function getSoapClient(): \SoapClient
    {
        return new \SoapClient('https://legacy_system.pl/Soap/WebService', []);
    }
}

✔️ It's required to split up an overcomplicated code to separate classes.

final class ApplicationService
{
    public function __construct(
        private OrderRepositoryInterface $orderRepository,
        private FormRepositoryInterface $formRepository,
        private FormApiInterface $formApi,
        private ChangeFormStatusService $changeFormStatusService
    ) {}

    public function changeFormStatus(int $orderId): void
    {
        $order = $this->orderRepository->getById($orderId);
        $form = $this->formRepository->getByOrderId($orderId);
        $status = $this->formApi->getStatusByOrderId($orderId);

        $this->changeFormStatusService->changeStatus($order, $form, $status);

        $this->formRepository->save($form);
        $this->orderRepository->save($order);
    }
}

final class ChangeFormStatusService
{
    public function changeStatus(Order $order, Form $form, string $formStatus): void
    {
        $status = FormStatus::createFromString($formStatus);
        $form->changeStatus($status);

        if ($form->isAccepted()) {
            $order->changeStatus(OrderStatus::paid());
        }
    }
}

final class ChangingFormStatusTest extends TestCase
{
    /**
     * @test
     */
    public function changing_a_form_status_to_accepted_changes_an_order_status_to_paid(): void
    {
        $order = new Order();
        $form = new Form();
        $status = 'accepted';
        $sut = new ChangeFormStatusService();

        $sut->changeStatus($order, $form, $status);

        self::assertTrue($form->isAccepted());
        self::assertTrue($order->isPaid());
    }

    /**
     * @test
     */
    public function changing_a_form_status_to_refused_not_changes_an_order_status(): void
    {
        $order = new Order();
        $form = new Form();
        $status = 'new';
        $sut = new ChangeFormStatusService();

        $sut->changeStatus($order, $form, $status);

        self::assertFalse($form->isAccepted());
        self::assertFalse($order->isPaid());
    }
}

However, ApplicationService probably should be tested by an integration test with only mocked FormApiInterface.

Trivial test

❌ Bad:

final class Customer
{
    public function __construct(private string $name) {}

    public function getName(): string
    {
        return $this->name;
    }

    public function setName(string $name): void
    {
        $this->name = $name;
    }
}

final class CustomerTest extends TestCase
{
    public function testSetName(): void
    {
        $customer = new Customer('Jack');

        $customer->setName('John');

        self::assertSame('John', $customer->getName());
    }
}

final class EventSubscriber
{
    public static function getSubscribedEvents(): array
    {
        return ['event' => 'onEvent'];
    }

    public function onEvent(): void
    {

    }
}

final class EventSubscriberTest extends TestCase
{
    public function testGetSubscribedEvents(): void
    {
        $result = EventSubscriber::getSubscribedEvents();

        self::assertSame(['event' => 'onEvent'], $result);
    }
}

❗️ Testing the code without any complicated logic is senseless, but also leads to fragile tests.

Fragile test

❌ Bad:

final class UserRepository
{
    public function __construct(
        private Connection $connection
    ) {}

    public function getUserNameByEmail(string $email): ?array
    {
        return $this
            ->connection
            ->createQueryBuilder()
            ->from('user', 'u')
            ->where('u.email = :email')
            ->setParameter('email', $email)
            ->execute()
            ->fetch();
    }
}

final class TestUserRepository extends TestCase
{
    public function testGetUserNameByEmail(): void
    {
        $email = '[email protected]';
        $connection = $this->createMock(Connection::class);
        $queryBuilder = $this->createMock(QueryBuilder::class);
        $result = $this->createMock(ResultStatement::class);
        $userRepository = new UserRepository($connection);
        $connection
            ->expects($this->once())
            ->method('createQueryBuilder')
            ->willReturn($queryBuilder);
        $queryBuilder
            ->expects($this->once())
            ->method('from')
            ->with('user', 'u')
            ->willReturn($queryBuilder);
        $queryBuilder
            ->expects($this->once())
            ->method('where')
            ->with('u.email = :email')
            ->willReturn($queryBuilder);
        $queryBuilder
            ->expects($this->once())
            ->method('setParameter')
            ->with('email', $email)
            ->willReturn($queryBuilder);
        $queryBuilder
            ->expects($this->once())
            ->method('execute')
            ->willReturn($result);
        $result
            ->expects($this->once())
            ->method('fetch')
            ->willReturn(['email' => $email]);

        $result = $userRepository->getUserNameByEmail($email);

        self::assertSame(['email' => $email], $result);
    }
}

❗️ Testing repositories in that way leads to fragile tests and then refactoring is tough. To test repositories write integration tests.

Test fixtures

❌ Bad:

final class InvalidTest extends TestCase
{
    private ?Subscription $subscription;

    public function setUp(): void
    {
        $this->subscription = new Subscription(new \DateTimeImmutable());
        $this->subscription->activate();
    }

    /**
     * @test
     */
    public function suspending_an_active_subscription_with_cannot_suspend_new_policy_is_possible(): void
    {
        $result = $this->subscription->suspend(new CannotSuspendNewSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function suspending_an_active_subscription_with_cannot_suspend_expired_policy_is_possible(): void
    {
        $result = $this->subscription->suspend(new CannotSuspendExpiredSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function suspending_a_new_subscription_with_cannot_suspend_new_policy_is_not_possible(): void
    {
        // Here we need to create a new subscription, it is not possible to change $this->subscription to a new subscription
    }
}

✔️ Good:

final class ValidTest extends TestCase
{
    /**
     * @test
     */
    public function suspending_an_active_subscription_with_cannot_suspend_new_policy_is_possible(): void
    {
        $sut = $this->createAnActiveSubscription();

        $result = $sut->suspend(new CannotSuspendNewSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function suspending_an_active_subscription_with_cannot_suspend_expired_policy_is_possible(): void
    {
        $sut = $this->createAnActiveSubscription();

        $result = $sut->suspend(new CannotSuspendExpiredSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertTrue($result);
    }

    /**
     * @test
     */
    public function suspending_a_new_subscription_with_cannot_suspend_new_policy_is_not_possible(): void
    {
        $sut = $this->createANewSubscription();

        $result = $sut->suspend(new CannotSuspendNewSubscriptionPolicy(), new \DateTimeImmutable());

        self::assertFalse($result);
    }

    private function createANewSubscription(): Subscription
    {
        return new Subscription(new \DateTimeImmutable());
    }

    private function createAnActiveSubscription(): Subscription
    {
        $subscription = new Subscription(new \DateTimeImmutable());
        $subscription->activate();
        return $subscription;
    }
}

• It's better to avoid a shared state between tests.
• To reuse elements between a few tests:
  • private factory methods - reusing in one class (like above)
  • Object mother - reusing in a few classes

General testing anti-patterns

Exposing private state

❌ Bad:

final class Customer
{
    private CustomerType $type;

    private DiscountCalculationPolicyInterface $discountCalculationPolicy;

    public function __construct()
    {
        $this->type = CustomerType::NORMAL();
        $this->discountCalculationPolicy = new NormalDiscountPolicy();
    }

    public function makeVip(): void
    {
        $this->type = CustomerType::VIP();
        $this->discountCalculationPolicy = new VipDiscountPolicy();
    }

    public function getCustomerType(): CustomerType
    {
        return $this->type;
    }

    public function getPercentageDiscount(): int
    {
        return $this->discountCalculationPolicy->getPercentageDiscount();
    }
}

final class InvalidTest extends TestCase
{
    public function testMakeVip(): void
    {
        $sut = new Customer();
        $sut->makeVip();

        self::assertSame(CustomerType::VIP(), $sut->getCustomerType());
    }
}

✔️ Good:

final class Customer
{
    private CustomerType $type;

    private DiscountCalculationPolicyInterface $discountCalculationPolicy;

    public function __construct()
    {
        $this->type = CustomerType::NORMAL();
        $this->discountCalculationPolicy = new NormalDiscountPolicy();
    }

    public function makeVip(): void
    {
        $this->type = CustomerType::VIP();
        $this->discountCalculationPolicy = new VipDiscountPolicy();
    }

    public function getPercentageDiscount(): int
    {
        return $this->discountCalculationPolicy->getPercentageDiscount();
    }
}

final class ValidTest extends TestCase
{
    /**
     * @test
     */
    public function a_vip_customer_has_a_25_percentage_discount(): void
    {
        $sut = new Customer();
        $sut->makeVip();

        self::assertSame(25, $sut->getPercentageDiscount());
    }
}

❗️ Adding additional production code (e.g. getter getCustomerType()) only to verify the state in tests is a bad practice. It should be verified by another domain significant value (in this case getPercentageDiscount()). Of course, sometimes it can be tough to find another way to verify the operation, and we can be forced to add additional production code to verify correctness in tests, but we should try to avoid that.

Leaking domain details

final class DiscountCalculator
{
    public function calculate(int $isVipFromYears): int
    {
        Assert::greaterThanEq($isVipFromYears, 0);
        return min(($isVipFromYears * 10) + 3, 80);
    }
}

❌ Bad:

final class InvalidTest extends TestCase
{
    /**
     * @dataProvider discountDataProvider
     */
    public function testCalculate(int $vipDaysFrom, int $expected): void
    {
        $sut = new DiscountCalculator();

        self::assertSame($expected, $sut->calculate($vipDaysFrom));
    }

    public function discountDataProvider(): array
    {
        return [
            [0, 0 * 10 + 3], //leaking domain details
            [1, 1 * 10 + 3],
            [5, 5 * 10 + 3],
            [8, 80]
        ];
    }
}

✔️ Good:

final class ValidTest extends TestCase
{
    /**
     * @dataProvider discountDataProvider
     */
    public function testCalculate(int $vipDaysFrom, int $expected): void
    {
        $sut = new DiscountCalculator();

        self::assertSame($expected, $sut->calculate($vipDaysFrom));
    }

    public function discountDataProvider(): array
    {
        return [
            [0, 3],
            [1, 13],
            [5, 53],
            [8, 80]
        ];
    }
}

ℹ️ Don't duplicate the production logic in tests. Just verify results by hardcoded values.

Mocking concrete classes

❌ Bad:

class DiscountCalculator
{
    public function calculateInternalDiscount(int $isVipFromYears): int
    {
        Assert::greaterThanEq($isVipFromYears, 0);
        return min(($isVipFromYears * 10) + 3, 80);
    }

    public function calculateAdditionalDiscountFromExternalSystem(): int
    {
        // get data from an external system to calculate a discount
        return 5;
    }
}

class OrderService
{
    public function __construct(private DiscountCalculator $discountCalculator) {}

    public function getTotalPriceWithDiscount(int $totalPrice, int $vipFromDays): int
    {
        $internalDiscount = $this->discountCalculator->calculateInternalDiscount($vipFromDays);
        $externalDiscount = $this->discountCalculator->calculateAdditionalDiscountFromExternalSystem();
        $discountSum = $internalDiscount + $externalDiscount;
        return $totalPrice - (int) ceil(($totalPrice * $discountSum) / 100);
    }
}

final class InvalidTest extends TestCase
{
    /**
     * @dataProvider orderDataProvider
     */
    public function testGetTotalPriceWithDiscount(int $totalPrice, int $vipDaysFrom, int $expected): void
    {
        $discountCalculator = $this->createPartialMock(DiscountCalculator::class, ['calculateAdditionalDiscountFromExternalSystem']);
        $discountCalculator->method('calculateAdditionalDiscountFromExternalSystem')->willReturn(5);
        $sut = new OrderService($discountCalculator);

        self::assertSame($expected, $sut->getTotalPriceWithDiscount($totalPrice, $vipDaysFrom));
    }

    public function orderDataProvider(): array
    {
        return [
            [1000, 0, 920],
            [500, 1, 410],
            [644, 5, 270],
        ];
    }
}

✔️ Good:

interface ExternalDiscountCalculatorInterface
{
    public function calculate(): int;
}

final class InternalDiscountCalculator
{
    public function calculate(int $isVipFromYears): int
    {
        Assert::greaterThanEq($isVipFromYears, 0);
        return min(($isVipFromYears * 10) + 3, 80);
    }
}

final class OrderService
{
    public function __construct(
        private InternalDiscountCalculator $discountCalculator,
        private ExternalDiscountCalculatorInterface $externalDiscountCalculator
    ) {}

    public function getTotalPriceWithDiscount(int $totalPrice, int $vipFromDays): int
    {
        $internalDiscount = $this->discountCalculator->calculate($vipFromDays);
        $externalDiscount = $this->externalDiscountCalculator->calculate();
        $discountSum = $internalDiscount + $externalDiscount;
        return $totalPrice - (int) ceil(($totalPrice * $discountSum) / 100);
    }
}

final class ValidTest extends TestCase
{
    /**
     * @dataProvider orderDataProvider
     */
    public function testGetTotalPriceWithDiscount(int $totalPrice, int $vipDaysFrom, int $expected): void
    {
        $externalDiscountCalculator = $this->createStub(ExternalDiscountCalculatorInterface::class);
        $externalDiscountCalculator->method('calculate')->willReturn(5);
        $sut = new OrderService(new InternalDiscountCalculator(), $externalDiscountCalculator);

        self::assertSame($expected, $sut->getTotalPriceWithDiscount($totalPrice, $vipDaysFrom));
    }

    public function orderDataProvider(): array
    {
        return [
            [1000, 0, 920],
            [500, 1, 410],
            [644, 5, 270],
        ];
    }
}

ℹ️ The necessity to mock a concrete class to replace a part of its behavior means that this class is probably too complicated and violates the Single Responsibility Principle.

Testing private methods

final class OrderItem
{
    public function __construct(private int $total) {}

    public function getTotal(): int
    {
        return $this->total;
    }
}

final class Order
{
    /**
     * @param OrderItem[] $items
     * @param int $transportCost
     */
    public function __construct(private array $items, private int $transportCost) {}

    public function getTotal(): int
    {
        return $this->getItemsTotal() + $this->transportCost;
    }

    private function getItemsTotal(): int
    {
        return array_reduce(
            array_map(fn (OrderItem $item) => $item->getTotal(), $this->items),
            fn (int $sum, int $total) => $sum += $total,
            0
        );
    }
}

❌ Bad:

final class InvalidTest extends TestCase
{
    /**
     * @test
     * @dataProvider ordersDataProvider
     */
    public function get_total_returns_a_total_cost_of_a_whole_order(Order $order, int $expectedTotal): void
    {
        self::assertSame($expectedTotal, $order->getTotal());
    }

    /**
     * @test
     * @dataProvider orderItemsDataProvider
     */
    public function get_items_total_returns_a_total_cost_of_all_items(Order $order, int $expectedTotal): void
    {
        self::assertSame($expectedTotal, $this->invokePrivateMethodGetItemsTotal($order));
    }

    public function ordersDataProvider(): array
    {
        return [
            [new Order([new OrderItem(20), new OrderItem(20), new OrderItem(20)], 15), 75],
            [new Order([new OrderItem(20), new OrderItem(30), new OrderItem(40)], 0), 90],
            [new Order([new OrderItem(99), new OrderItem(99), new OrderItem(99)], 9), 306]
        ];
    }

    public function orderItemsDataProvider(): array
    {
        return [
            [new Order([new OrderItem(20), new OrderItem(20), new OrderItem(20)], 15), 60],
            [new Order([new OrderItem(20), new OrderItem(30), new OrderItem(40)], 0), 90],
            [new Order([new OrderItem(99), new OrderItem(99), new OrderItem(99)], 9), 297]
        ];
    }

    private function invokePrivateMethodGetItemsTotal(Order &$order): int
    {
        $reflection = new \ReflectionClass(get_class($order));
        $method = $reflection->getMethod('getItemsTotal');
        $method->setAccessible(true);
        return $method->invokeArgs($order, []);
    }
}

✔️ Good:

final class ValidTest extends TestCase
{
    /**
     * @test
     * @dataProvider ordersDataProvider
     */
    public function get_total_returns_a_total_cost_of_a_whole_order(Order $order, int $expectedTotal): void
    {
        self::assertSame($expectedTotal, $order->getTotal());
    }

    public function ordersDataProvider(): array
    {
        return [
            [new Order([new OrderItem(20), new OrderItem(20), new OrderItem(20)], 15), 75],
            [new Order([new OrderItem(20), new OrderItem(30), new OrderItem(40)], 0), 90],
            [new Order([new OrderItem(99), new OrderItem(99), new OrderItem(99)], 9), 306]
        ];
    }
}

❗️ Tests should only verify public API.

Time as a volatile dependency

ℹ️ The time is a volatile dependency because it is non-deterministic. Each invocation returns a different result.

❌ Bad:

final class Clock
{
    public static \DateTime|null $currentDateTime = null;

    public static function getCurrentDateTime(): \DateTime
    {
        if (null === self::$currentDateTime) {
            self::$currentDateTime = new \DateTime();
        }

        return self::$currentDateTime;
    }

    public static function set(\DateTime $dateTime): void
    {
        self::$currentDateTime = $dateTime;
    }

    public static function reset(): void
    {
        self::$currentDateTime = null;
    }
}

final class Customer
{
    private \DateTime $createdAt;

    public function __construct()
    {
        $this->createdAt = Clock::getCurrentDateTime();
    }

    public function isVip(): bool
    {
        return $this->createdAt->diff(Clock::getCurrentDateTime())->y >= 1;
    }
}

final class InvalidTest extends TestCase
{
    /**
     * @test
     */
    public function a_customer_registered_more_than_a_one_year_ago_is_a_vip(): void
    {
        Clock::set(new \DateTime('2019-01-01'));
        $sut = new Customer();
        Clock::reset(); // you have to remember about resetting the shared state

        self::assertTrue($sut->isVip());
    }

    /**
     * @test
     */
    public function a_customer_registered_less_than_a_one_year_ago_is_not_a_vip(): void
    {
        Clock::set((new \DateTime())->sub(new \DateInterval('P2M')));
        $sut = new Customer();
        Clock::reset(); // you have to remember about resetting the shared state

        self::assertFalse($sut->isVip());
    }
}

✔️ Good:

interface ClockInterface
{
    public function getCurrentTime(): \DateTimeImmutable;
}

final class Clock implements ClockInterface
{
    private function __construct()
    {
    }

    public static function create(): self
    {
        return new self();
    }

    public function getCurrentTime(): \DateTimeImmutable
    {
        return new \DateTimeImmutable();
    }
}

final class FixedClock implements ClockInterface
{
    private function __construct(private \DateTimeImmutable $fixedDate) {}

    public static function create(\DateTimeImmutable $fixedDate): self
    {
        return new self($fixedDate);
    }

    public function getCurrentTime(): \DateTimeImmutable
    {
        return $this->fixedDate;
    }
}

final class Customer
{
    private \DateTimeImmutable $createdAt;

    public function __construct(\DateTimeImmutable $createdAt)
    {
        $this->createdAt = $createdAt;
    }

    public function isVip(\DateTimeImmutable $currentDate): bool
    {
        return $this->createdAt->diff($currentDate)->y >= 1;
    }
}

final class ValidTest extends TestCase
{
    /**
     * @test
     */
    public function a_customer_registered_more_than_a_one_year_ago_is_a_vip(): void
    {
        $sut = new Customer(FixedClock::create(new \DateTimeImmutable('2019-01-01'))->getCurrentTime());

        self::assertTrue($sut->isVip(FixedClock::create(new \DateTimeImmutable('2020-01-02'))->getCurrentTime()));
    }

    /**
     * @test
     */
    public function a_customer_registered_less_than_a_one_year_ago_is_not_a_vip(): void
    {
        $sut = new Customer(FixedClock::create(new \DateTimeImmutable('2019-01-01'))->getCurrentTime());

        self::assertFalse($sut->isVip(FixedClock::create(new \DateTimeImmutable('2019-05-02'))->getCurrentTime()));
    }
}

ℹ️ The time and random numbers should not be generated directly in the domain code. To test behavior we must have deterministic results, so we need to inject these values into a domain object like in the example above.

100% Test Coverage shouldn't be the goal

100% Coverage is not the goal or even is undesirable because if there is 100% coverage, tests probably will be very fragile, which means refactoring will be very hard. Mutation testing gives better feedback about the quality of tests. Read more

Recommended books

• Test Driven Development: By Example / Kent Beck - the classic
• Unit Testing Principles, Practices, and Patterns / Vladimir Khorikov - the best book about tests I've ever read