Symfony\Component\DependencyInjection\ContainerBuilder::getDefinition PHP Method

    public function getDefinition($id)
    {
        $id = strtolower($id);

        if (!isset($this->definitions[$id])) {
            throw new ServiceNotFoundException($id);
        }

        return $this->definitions[$id];
    }

 public function process(ContainerBuilder $container)
 {
     if (!$container->hasDefinition('form.extension')) {
         return;
     }
     $definition = $container->getDefinition('form.extension');
     // Builds an array with service IDs as keys and tag aliases as values
     $types = array();
     foreach ($container->findTaggedServiceIds('form.type') as $serviceId => $tag) {
         // The following if-else block is deprecated and will be removed
         // in Symfony 3.0
         // Deprecation errors are triggered in the form registry
         if (isset($tag[0]['alias'])) {
             $types[$tag[0]['alias']] = $serviceId;
         } else {
             $types[$serviceId] = $serviceId;
         }
         // Support type access by FQCN
         $serviceDefinition = $container->getDefinition($serviceId);
         $types[$serviceDefinition->getClass()] = $serviceId;
     }
     $definition->replaceArgument(1, $types);
     $typeExtensions = array();
     foreach ($container->findTaggedServiceIds('form.type_extension') as $serviceId => $tag) {
         $alias = isset($tag[0]['alias']) ? $tag[0]['alias'] : $serviceId;
         $typeExtensions[$alias][] = $serviceId;
     }
     $definition->replaceArgument(2, $typeExtensions);
     // Find all services annotated with "form.type_guesser"
     $guessers = array_keys($container->findTaggedServiceIds('form.type_guesser'));
     $definition->replaceArgument(3, $guessers);
 }