muzich/src/Muzich/CoreBundle/DependencyInjection/CoreExtension.php

<?php

namespace Muzich\CoreBundle\DependencyInjection;

use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Definition;

class CoreExtension extends Extension
{

  public function load(array $config, ContainerBuilder $container) {
      $definition = new Definition('Muzich\CoreBundle\Extension\MyTwigExtension');
      // this is the most important part. Later in the startup process TwigBundle
      // searches through the container and registres all services taged as twig.extension.
      $definition->addTag('twig.extension');
      $container->setDefinition('my_twig_extension', $definition);
  }

  /**
   * Was necessary in previous Symfony2 PR releases.
   * Symfony2 calls `load` method automatically now.
   *
   * public function getAlias() {
   *     return 'hello'; // that's how we'll call this extension in configuration files
   * }
   */
}