As seen in your sample code the class is located in another namespace :
<?php
namespace Acme\MyBundle\Service;
use Symfony\Component\HttpFoundation\File\File;
use Imagine\Image\ImagineInterface;
use Imagine\Image\BoxInterface;
use Imagine\Image\Point;
use Imagine\Image\Box;
class ImagineResizer {
//-- rest of code
}
To use a class in another namespace you need to point out where the file is :
First include the class (manual or with autoloading)
Then u can create an instance in 2 ways. First way with the use
-keyword
use Acme\MyBundle\Service\ImageResizer;
$object = new ImageResizer();
Or point to the class absolute :
$object = new \Acme\MyBundle\Service\ImageResizer();