Symfony2 Twig extension

I have some problem to implement twig extension. I need to create my own filter (a substr filter). So I have thought to use twig extension.

I created a folder named "Extension" on MyApp\Bundle\WebsiteBundle\ and file : WebsiteExtension.php

namespace Bundle\WebsiteBundle\Extension;



class WebsiteExtension extends \Twig_Extension {



    public function getFilters() {

        return array(

            'substr'  => new \Twig_Filter_Method($this, 'substrFilter'),

        );

    }



    public function substrFilter($sentence, $start, $end) {

        return substr($sentence, $start, $end);

    }



    public function getName()

    {

        return 'website_extension';

    }

}

Then on my config.yml on /app/config

services:

    Website.twig.extension:

        class: MyApp\Bundle\WebsiteBundle\Extension\WebsiteExtension

        tags:

            - { name: twig.extension }

But error occurs :

"Fatal error: Class 'MyApp\Bundle\WebsiteBundle\Extension\WebsiteExtension' not found in .../appDevDebugProjectContainer.php on line 1391"

It's strange because file and class exists ... I have missed something but what ?

Source: Tips4all, CCNA FINAL EXAM