a.php

<?

// src/Acme/HelloBundle/DependencyInjection/AcmeHelloExtension.php
namespace Acme\HelloBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\DependencyInjection\ConfigurableExtension;

class AcmeHelloExtension extends ConfigurableExtension
{
    // note that this method is called loadInternal and not load
    protected function loadInternal(array $mergedConfig, ContainerBuilder $container)
    {
        // ...
    }
}