Symfony\Component\HttpKernel\Bundle\Bundle::getContainerExtension PHP Method

    public function getContainerExtension()
    {
        if (null === $this->extension) {
            $extension = $this->createContainerExtension();

            if (null !== $extension) {
                if (!$extension instanceof ExtensionInterface) {
                    throw new \LogicException(sprintf('Extension %s must implement Symfony\Component\DependencyInjection\Extension\ExtensionInterface.', get_class($extension)));
                }

                // check naming convention
                $basename = preg_replace('/Bundle$/', '', $this->getName());
                $expectedAlias = Container::underscore($basename);

                if ($expectedAlias != $extension->getAlias()) {
                    throw new \LogicException(sprintf(
                        'Users will expect the alias of the default extension of a bundle to be the underscored version of the bundle name ("%s"). You can override "Bundle::getContainerExtension()" if you want to use "%s" or another alias.',
                        $expectedAlias, $extension->getAlias()
                    ));
                }

                $this->extension = $extension;
            } else {
                $this->extension = false;
            }
        }

        if ($this->extension) {
            return $this->extension;
        }
    }

 public static function addBundle(Bundle $bundle)
 {
     $extension = $bundle->getContainerExtension();
     if ($extension) {
         self::$extensions[] = $extension;
     }
     self::$bundlePaths[] = $bundle->getPath();
 }