Deprecated
Check out https://github.com/BlockHorizons/Fireworks/ for an up to date firework plugin, including a simple API!
fireworks 
Adds Fireworks to Pocketmine
Its simply adding firework items and entities into PocketMine-MP servers.
You can launch any firework from the creative inventory AND custom ones - just like in vanilla MCPE.
How to get a firework
Either, give a firework via gamemode 1, or /give command. For generating the /give command i suggest using: https://mcstacker.bimbimma.com/mcstacker1.10.php and modifying the command to work with PocketMine-MP. This is not easy. No need to explain further.
The easier way is to use the code to create custom fireworks. See Create a custom firework for that.
Create a custom firework
This is sample code to explain the "API"-alike part of the plugin, giving 2 custom fireworks:
Without special explosions:
$data = new FireworksData();
$data->flight = 2;//flight time. default is 1
$firework = new Fireworks();
$nbt = Fireworks::ToNbt($data);//create the compound tag
$firework->setNamedTag($nbt);//modify the item's compound tag
$event->getPlayer()->getInventory()->addItem($firework);//give the item to a player (this case: PlayerJoinEvent $event)
With special explosions
$explosion = new FireworksExplosion();//init an explosion
$explosion->fireworkColor = [4, 4, 4];//set the color to custom colors
$explosion->fireworkFade = [5, 5, 5];//set the fade color to custom colors
$explosion->fireworkFlicker = true;//enable the flickering
$explosion->fireworkTrail = false;//enable the trail
$explosion->fireworkType = 4;//set the shape/type of the firework
$data = new FireworksData();//create the generic data
$data->flight = 2;
$data->explosions[] = $explosion;//add the explosion
$firework = new Fireworks();
$nbt = Fireworks::ToNbt($data);
$firework->setNamedTag($nbt);
$event->getPlayer()->getInventory()->addItem($firework);