oliverdavies.uk/src/AppBundle/DependencyInjection/AppExtension.php

<?php

namespace AppBundle\DependencyInjection;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\DependencyInjection\Extension;
use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
use Symfony\Component\Config\FileLocator;

class AppExtension extends Extension
{
    /**
     * {@inheritdoc}
     */
    public function load(array $configs, ContainerBuilder $container)
    {
        $loader = new YamlFileLoader($container, new FileLocator(__DIR__ . '/../Resources/config'));
        $loader->load('services.yml');
    }
}