cd /path/to/php/extention/source/directory
https://github.com/xymak/php-smartcrop-extension.git
cd php-smartcrop-extension
phpize
./configure --prefix='/path/to/php/directory' --with-php-config='/path/to/php/config'
make && make install
echo "extension=smartcrop.so" >> /path/to/etc/php/php.d/smartcorp.ini

<?php
//Convert to a image to gd resource
$im = imagecreatefromstring(file_get_contents('/path/to/a/image'));
//Target Size
$width = 400;
$height = 400;
//Call smartcrop function
$cim = smartcrop($im,$width,$height);