Zan Baldwin
Monday, 11th March, 2019

What is Symfony?


Symfony is a toolbox of (over 50) decoupled and reusable software components on which the best PHP applications are built, with outstanding documentation, comprehensive test coverage and contribution guidelines.

Examples of components include:

Projects that are using Symfony components include:


The framework is a methodology (or "assembly guide") for web applications built on top of the components as glue-code to bring them all together in a structured approach. It efficiently and effectively guarantees stability, maintainability and upgradability of complex applications.


The philosophy of Symfony is to embrace and promote professionalism, best practices, standardization and interoperability of applications.


Symfony is a huge and diverse community of over 600,000 developers across 120 countries. It is committed to fostering an open and welcoming environment for everyone, with an appropriate Code of Conduct, Diversity Initiative, support system, user groups, and 9 conferences a year spread across 4 continents.


A Symfony application has a very simple file structure.

├ bin                Binary executables
│  └ console         Entrypoint to Symfony via CLI (equivalent to Drush or similar)
├ config             All configuration files go in here: services, routing, package config, etc.
│  └ …               By default, most files in this directory are written in YAML
├ public             Webroot directory
│  └ index.php       Entrypoint to Symfony via HTTP
├ src                All project source code goes in here
│  ├ Controller
│  │  └ .gitignore
│  └ Kernel.php      Core of Symfony; sets up bundles, config, and provides service container
├ var                Variable data: logs, cache, compiled code
├ vendor             Third-party package dependencies
├ .env               Default values for project-specific environment variables
├ .gitignore
├ composer.json
├ composer.lock
└ symfony.lock       Similar to composer.lock, but for Symfony Flex

Flex and Recipes

Modern Symfony (v4.0+) relies heavily on Composer. Flex is a Symfony-specific Composer plugin that provides extra steps ("recipes") to execute when installing specific, third-party packages.

Example Recipe (Doctrine)

The recipe executed during the installation of the package doctrine/doctrine-bundleperforms three steps:

  1. Registers the Doctrine bundle with your application's kernel.
  2. Creates the file config/packages/doctrine.yaml
  3. Adds the environment variable DATABASE_URL to your .env file.

These recipes feel like magic at first, but follow predetermined steps defined in the symfony/recipes repository.

They set up sensible defaults so that a package can be installed and used without having to invoke some arcane Google-fu to figure out what configuration options go where.

Magic (programming), ˈmadʒɪk

An informal term, often carrying bad connotations, for heavy abstraction that hides the true behaviour of code. The action of such abstractions is described as being done "automagically".

Many advanced process in Symfony are abstracted away from the developer (DX initiative) and can seem like magic, but are logical and customisable once you start exploring.

Getting Started

The skeleton repository for setting up a new Symfony project (git:// contains just two files: composer.json and composer.lock.

Running composer install will:

  1. Install the project's dependencies to the vendor/ directory. To start off with, our application has 5 direct dependencies:
  2. The symfony/flex package is installed and the custom Composer plugin will run, fetching and executing any recipes that exist for the installed dependencies.

The recipe for the symfony/framework-bundle package bootstraps the project structure, resulting in the file structure we saw before.


Plugins, modules, extensions… etc.

They can provide any functionality the main application can, bundled into a reusable, distributable package. By default, only the FrameworkBundle is included.

Bundles used to be first-class citizens in Symfony, but they are now abstracted away. You don't need to know much about bundles except they provide functionality and, with Flex, most of them automagically configure themselves with sensible defaults when they are installed through Composer.

Dependency Injection

Every object-oriented PHP project contains useful objects that do work: the Twig object renders templates and the Monolog object logs messages. These objects are called services and live inside a centralised registry called a service container.

Services are the most important part of Symfony. Using the previous examples, the Twig bundle configures and registers the templating service with the service container, and the Monolog bundle configures and registers the logger service with the service container.


        class: 'App\Email\Transport'
        arguments: [ '%smtp_server%', '%username%', '%password%' ]

        class: 'App\Email\Emailer'
        arguments: [ '@email_transport_service' ]
            - [ 'setLogger', [ '@logger' ]]

By default, configuration in Symfony is written in YAML. It's also possible to write configuration in XML or pure PHP if prefered. There is no performance penalty for using any format, configuration is parsed and compiled before being dumped as pure PHP in the cache.

Symfony is also extensible enough for custom config loaders to be written for other configuration formats such as JSON or TOML if you happen to be a masochist.

Service Definitions

Service definitions describe how a service (PHP object) should be constructed, referencing other services @service_name and configuration %parameter%'s. Other configuration options for service definitions include:

Service Configuration

Using the example above, requesting the service my_emailer_service from the service container will return an instantiated class of type App\Email\Emailer, with the service email_transport_service injected into its constructor (which has, in turn, been constructed with the appropriate parameters injected into that constructor), and the service logger injected into its setLogger() method.

Application Service Configuration

The default settings that come with symfony/skeleton (or more precisely, the symfony/framework-bundle recipe that gets executed when installing symfony/skeleton) are:

# This file is the entry point to configure your own services.
# Files in the packages/ subdirectory configure your dependencies.

# Put parameters here that don't need to change on each machine where the app is deployed

    # default configuration for services in *this* file
        autowire: true      # Automatically injects dependencies in your services.
        autoconfigure: true # Automatically registers your services as commands, event subscribers, etc.

    # makes classes in src/ available to be used as services
    # this creates a service per class whose id is the fully-qualified class name
        resource: '../src/*'
        exclude: '../src/{DependencyInjection,Entity,Migrations,Tests,Kernel.php}'

    # controllers are imported separately to make sure services can be injected
    # as action arguments even if you don't extend any base controller class
        resource: '../src/Controller'
        tags: ['controller.service_arguments']

    # add more service definitions when explicit configuration is needed
    # please note that last definitions always *replace* previous ones

Auto Service Loading

Automatic service loading allows entire namespaces/directories to be scanned and automatically added as services to the service container, without the need to manually specify service definitions for every single class.


    # makes classes in src/ available to be used as services
    # this creates a service per class whose id is the fully-qualified class name
        resource: '../src/*'
        exclude: '../src/{DependencyInjection,Entity,Migrations,Tests,Kernel.php}'


Each service defined in the service container must have a unique name (its service ID); it's convention to use the class name.

Autowiring is a service container feature (enabled by default) that tries to automatically configure the value to inject for every service's constructor arguments that has not been manually defined (as is the case with automatic service loading). Each service's constructor is scanned for argument type-hints; if the type-hint exactly matches the ID of a service registered with the service container, it is used. This is why using the class name as the service ID is the convention.


    # Service identifier same as FQCN, no need to specify "class" option.
        arguments: [ '%smtp_server%', '%username%', '%password%' ]

        # Arguments no longer have to be specified manually.
        autowire: true
        # Calls can also be autowired using the @required annotation.
            - [ 'setLogger', [ '@logger' ]]

    # Alternatively, apply autowiring to all services defined in this file.
        # Autowire: automatically inject dependencies in your services.
        autowire: true

If an interface is implemented by one (and only one) concrete class, the service container is smart enough to realise that when you type-hint an interface you mean the concrete class - it will register the interface as an alias of the service for the concrete class so that autowiring still works. If more than one concrete class implements the interface, manual configuration is required.

Auto Configuration

Auto configuration is another service container feature that will apply generic configuration to all services that implement a specific interface.

Custom auto configuration is not often used in applications - its main use is for Symfony to automatically find, register and use classes you create.


    # Default configuration for services in *this* file.
        # Automatically registers your services as commands, event subscribers, etc.
        autoconfigure: true
<?php declare(strict_types=1);

namespace App;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\HttpKernel\Kernel as BaseKernel;

class Kernel extends BaseKernel
    // ...

    protected function build(ContainerBuilder $container): void
        $container->addCompilerPass(new MyCompilerPassThatHandlesTaggedServices);

For example, making an event listener implement EventSubscriberInterface results in Symfony automatically registering and executing your event listener with no additional configuration needed.

<?php declare(strict_types=1);

namespace App\Listener;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\Exception\NotAcceptableHttpException;
use Symfony\Component\HttpKernel\KernelEvents;

class JsonListener implements EventSubscriberInterface
    /** {@inheritdoc} */
    public static function getSubscribedEvents(): array
        return [
            KernelEvents::REQUEST => ['checkContentTypeIsJson', 34],
    public function checkContentTypeIsJson(GetResponseEvent $event): void
        $request = $event->getRequest();
        if ($request->headers->get('Content-Type') !== 'application/json') {
            throw new NotAcceptableHttpException('This is an API, fool!');


Fire-and-forget: default configuration is enough 95% of the time. Just create classes and type hint everything.


Eventually, all that configuration gets "compiled" into executable, raw PHP files.

Compilation means YAML is not parsed on each request.


        autowire: true
        public: false

            $privateKey: '@oauth2.private_key'
            $encryptionKey: '%env(APP_SECRET)%'

Here be dragons:


use Symfony\Component\DependencyInjection\Argument\RewindableGenerator;
use Symfony\Component\DependencyInjection\Exception\RuntimeException;

// This file has been auto-generated by the Symfony Dependency Injection Component for internal use.
// Returns the private 'League\OAuth2\Server\AuthorizationServer' shared autowired service.

$this->privates['League\OAuth2\Server\AuthorizationServer'] =
$instance = new \League\OAuth2\Server\AuthorizationServer(
        ?? $this->load('getClientRepository2Service.php')),
        ?? $this->load('getAccessTokenRepositoryService.php')),
        ?? $this->load('getScopeRepositoryService.php')),
    new \League\OAuth2\Server\CryptKey(
$a = ($this->private['App\OAuth2\Repository\RefreshTokenRepository']
    ?? $this->load('getRefreshTokenRepositoryService.php'));
$b = new \League\OAuth2\Server\Grant\RefreshTokenGrant($a);
$c = new \DateInterval($this->getEnv('OAUTH2_REFRESH_TOKEN_DURATION'));
$d = new \DateInterval($this->getEnv('OAUTH2_ACCESS_TOKEN_DURATION'));
$e = new \League\OAuth2\Server\Grant\PasswordGrant(
        ?? $this->load('getUserRepository2Service.php')),
$f = new \League\OAuth2\Server\Grant\AuthCodeGrant(
        ?? $this->load('getAuthCodeRepositoryService.php')),
    new \DateInterval($this->getEnv('OAUTH2_AUTH_CODE_DURATION'))
$instance->enableGrantType($b, $d);
$instance->enableGrantType(new \League\OAuth2\Server\Grant\ClientCredentialsGrant(), $d);
$instance->enableGrantType($e, $d);
$instance->enableGrantType($f, $d);
return $instance;


The main entry point for application logic.

Controllers are what get triggered when a route is matched. I have no idea what the equivalent in Drupal is because I wasn't listening as much as I should have.


<?php declare(strict_types=1);

namespace App\Controller;

use App\Form\Type\User\EditUserType;
use Doctrine\ORM\EntityManagerInterface;
use Symfony\Bundle\FrameworkBundle\Controller\ControllerTrait;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

class UserController
    use ControllerTrait;
    private $em;
    public function __construct(EntityManagerInterface $em)
        $this->em = $em;
    public function editAction(Request $request, int $userId): Response
        if (null === $user = $this->em->findById(User::class, $userId)) {
            throw new NotFoundHttpException;
        $form = $this->createForm(EditUserType::class, $user);
        if ($form->isSubmitted() && $form->isValid()) {
            // Do stuff to update the user...
        return $this->render('edit_user.html.twig', [
            'user' => $user,
            'form' => $form->createView(),


<?php declare(strict_types=1);

namespace App\Controller\Webhook;

use App\Form\Type\WebhookType;
use App\Repository\WebhookRepository;
use App\Response\ApiResponse;
use App\Response\ApiResponseInterface;
use App\Validator\ValidationHttpException;
use Ramsey\Uuid\UuidInterface;
use Symfony\Bundle\FrameworkBundle\Controller\ControllerTrait;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

class UpdateWebhookAction
    use ControllerTrait;

    private $repository;
    public function __construct(WebhookRepository $repository)
        $this->repository = $repository;
    public function __invoke(Request $request, UuidInterface $webookUuid): ApiResponseInterface
        if (null === $webhook = $this->repository->find($webhookUuid)) {
            throw new NotFoundHttpException;
        $form = $this->createForm(WebhookType::class, $webhook);
        if ($form->isSubmitted() && $form->isValid()) {
            $webhook = $this->repository->update($webhook, $form->getData());
            return new ApiResponse($webhook);
        throw ValidationHttpException::fromForm($form);

The responder will then determine how this data structure should be converted into a HTTP response according to the MIME type requested by the end-user.