The Architecture

You are my hero! Who would have thought that you would still be here after the first three parts? Your efforts will be well rewarded soon. The first three parts didn’t look too deeply at the architecture of the framework. Because it makes Symfony stand apart from the framework crowd, let’s dive into the architecture now.

Understanding the Directory Structure

The directory structure of a Symfony application is rather flexible, but the recommended structure is as follows:

The application configuration, templates and translations.
The project’s PHP code.
The third-party dependencies.
The web root directory.

The web/ Directory

The web root directory is the home of all public and static files like images, stylesheets, and JavaScript files. It is also where each front controller lives, such as the production controller shown here:

// web/app.php
require_once __DIR__.'/../app/bootstrap.php.cache';
require_once __DIR__.'/../app/AppKernel.php';

use Symfony\Component\HttpFoundation\Request;

$kernel = new AppKernel('prod', false);
$request = Request::createFromGlobals();
$response = $kernel->handle($request);

The controller first bootstraps the application using a kernel class (AppKernel in this case). Then, it creates the Request object using the PHP’s global variables and passes it to the kernel. The last step is to send the response contents returned by the kernel back to the user.

The app/ Directory

The AppKernel class is the main entry point of the application configuration and as such, it is stored in the app/ directory.

This class must implement two methods:

Must return an array of all bundles needed to run the application, as explained in the next section.
Loads the application configuration (more on this later).

Autoloading is handled automatically via Composer, which means that you can use any PHP class without doing anything at all! All dependencies are stored under the vendor/ directory, but this is just a convention. You can store them wherever you want, globally on your server or locally in your projects.

Understanding the Bundle System

This section introduces one of the greatest and most powerful features of Symfony, the bundle system.

A bundle is kind of like a plugin in other software. So why is it called a bundle and not a plugin? This is because everything is a bundle in Symfony, from the core framework features to the code you write for your application.

All the code you write for your application is organized in bundles. In Symfony speak, a bundle is a structured set of files (PHP files, stylesheets, JavaScripts, images, ...) that implements a single feature (a blog, a forum, ...) and which can be easily shared with other developers.

Bundles are first-class citizens in Symfony. This gives you the flexibility to use pre-built features packaged in third-party bundles or to distribute your own bundles. It makes it easy to pick and choose which features to enable in your application and optimize them the way you want. And at the end of the day, your application code is just as important as the core framework itself.

Symfony already includes an AppBundle that you may use to start developing your application. Then, if you need to split the application into reusable components, you can create your own bundles.

Registering a Bundle

An application is made up of bundles as defined in the registerBundles() method of the AppKernel class. Each bundle is a directory that contains a single Bundle class that describes it:

// app/AppKernel.php
public function registerBundles()
    $bundles = array(
        new Symfony\Bundle\FrameworkBundle\FrameworkBundle(),
        new Symfony\Bundle\SecurityBundle\SecurityBundle(),
        new Symfony\Bundle\TwigBundle\TwigBundle(),
        new Symfony\Bundle\MonologBundle\MonologBundle(),
        new Symfony\Bundle\SwiftmailerBundle\SwiftmailerBundle(),
        new Symfony\Bundle\DoctrineBundle\DoctrineBundle(),
        new Symfony\Bundle\AsseticBundle\AsseticBundle(),
        new Sensio\Bundle\FrameworkExtraBundle\SensioFrameworkExtraBundle(),
        new AppBundle\AppBundle();

    if (in_array($this->getEnvironment(), array('dev', 'test'))) {
        $bundles[] = new Symfony\Bundle\WebProfilerBundle\WebProfilerBundle();
        $bundles[] = new Sensio\Bundle\DistributionBundle\SensioDistributionBundle();
        $bundles[] = new Sensio\Bundle\GeneratorBundle\SensioGeneratorBundle();

    return $bundles;

In addition to the AppBundle that was already talked about, notice that the kernel also enables other bundles that are part of Symfony, such as FrameworkBundle, DoctrineBundle, SwiftmailerBundle and AsseticBundle.

Configuring a Bundle

Each bundle can be customized via configuration files written in YAML, XML, or PHP. Have a look at this sample of the default Symfony configuration:

# app/config/config.yml
    - { resource: parameters.yml }
    - { resource: security.yml }
    - { resource: services.yml }

    #esi:             ~
    #translator:      { fallback: "%locale%" }
    secret:          "%secret%"
        resource: "%kernel.root_dir%/config/routing.yml"
        strict_requirements: "%kernel.debug%"
    form:            true
    csrf_protection: true
    validation:      { enable_annotations: true }
    templating:      { engines: ['twig'] }
    default_locale:  "%locale%"
    trusted_proxies: ~
    session:         ~

# Twig Configuration
    debug:            "%kernel.debug%"
    strict_variables: "%kernel.debug%"

# Swift Mailer Configuration
    transport: "%mailer_transport%"
    host:      "%mailer_host%"
    username:  "%mailer_user%"
    password:  "%mailer_password%"
    spool:     { type: memory }

# ...

Each first level entry like framework, twig and swiftmailer defines the configuration for a specific bundle. For example, framework configures the FrameworkBundle while swiftmailer configures the SwiftmailerBundle.

Each environment can override the default configuration by providing a specific configuration file. For example, the dev environment loads the config_dev.yml file, which loads the main configuration (i.e. config.yml) and then modifies it to add some debugging tools:

# app/config/config_dev.yml
    - { resource: config.yml }

    router:   { resource: "%kernel.root_dir%/config/routing_dev.yml" }
    profiler: { only_exceptions: false }

    toolbar: true
    intercept_redirects: false

# ...

Extending a Bundle

In addition to being a nice way to organize and configure your code, a bundle can extend another bundle. Bundle inheritance allows you to override any existing bundle in order to customize its controllers, templates, or any of its files.

Logical File Names

When you want to reference a file from a bundle, use this notation: @BUNDLE_NAME/path/to/file; Symfony will resolve @BUNDLE_NAME to the real path to the bundle. For instance, the logical path @AppBundle/Controller/DefaultController.php would be converted to src/AppBundle/Controller/DefaultController.php, because Symfony knows the location of the AppBundle.

Logical Controller Names

For controllers, you need to reference actions using the format BUNDLE_NAME:CONTROLLER_NAME:ACTION_NAME. For instance, AppBundle:Default:index maps to the indexAction method from the AppBundle\Controller\DefaultController class.

Extending Bundles

If you follow these conventions, then you can use bundle inheritance to override files, controllers or templates. For example, you can create a bundle - NewBundle - and specify that it overrides AppBundle. When Symfony loads the AppBundle:Default:index controller, it will first look for the DefaultController class in NewBundle and, if it doesn’t exist, then look inside AppBundle. This means that one bundle can override almost any part of another bundle!

Do you understand now why Symfony is so flexible? Share your bundles between applications, store them locally or globally, your choice.

Using Vendors

Odds are that your application will depend on third-party libraries. Those should be stored in the vendor/ directory. You should never touch anything in this directory, because it is exclusively managed by Composer. This directory already contains the Symfony libraries, the SwiftMailer library, the Doctrine ORM, the Twig templating system and some other third party libraries and bundles.

Understanding the Cache and Logs

Symfony applications can contain several configuration files defined in several formats (YAML, XML, PHP, etc.) Instead of parsing and combining all those files for each request, Symfony uses its own cache system. In fact, the application configuration is only parsed for the very first request and then compiled down to plain PHP code stored in the app/cache/ directory.

In the development environment, Symfony is smart enough to update the cache when you change a file. But in the production environment, to speed things up, it is your responsibility to clear the cache when you update your code or change its configuration. Execute this command to clear the cache in the prod environment:

$ php app/console cache:clear --env=prod

When developing a web application, things can go wrong in many ways. The log files in the app/logs/ directory tell you everything about the requests and help you fix the problem quickly.

Using the Command Line Interface

Each application comes with a command line interface tool (app/console) that helps you maintain your application. It provides commands that boost your productivity by automating tedious and repetitive tasks.

Run it without any arguments to learn more about its capabilities:

$ php app/console

The --help option helps you discover the usage of a command:

$ php app/console router:debug --help

Final Thoughts

Call me crazy, but after reading this part, you should be comfortable with moving things around and making Symfony work for you. Everything in Symfony is designed to get out of your way. So, feel free to rename and move directories around as you see fit.

And that’s all for the quick tour. From testing to sending emails, you still need to learn a lot to become a Symfony master. Ready to dig into these topics now? Look no further - go to the official The Book and pick any topic you want.