The Big Picture

Actions and Controllers

Open the src/AppBundle/Controller/DefaultController.php file and you'll see the following code (for now, don't look at the @Route configuration because that will be explained in the next section):

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namespace AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\Routing\Annotation\Route;

class DefaultController extends Controller
{
    /**
     * @Route("/", name="homepage")
     */
    public function indexAction()
    {
        return $this->render('default/index.html.twig', [
            // ...
        ]);
    }
}

In Symfony applications, controllers are usually PHP classes whose names are suffixed with the Controller word. In this example, the controller is called Default and the PHP class is called DefaultController.

The methods defined in a controller are called actions, they are usually associated with one URL of the application and their names are suffixed with Action. In this example, the Default controller has only one action called index and defined in the indexAction() method.

Actions are usually very short - around 10-15 lines of code - because they just call other parts of the application to get or generate the needed information and then they render a template to show the results to the user.

In this example, the index action is practically empty because it doesn't need to call any other method. The action just renders a template to welcome you to Symfony.

Routing

Symfony routes each request to the action that handles it by matching the requested URL against the paths configured by the application. Open again the src/AppBundle/Controller/DefaultController.php file and take a look at the three lines of code above the indexAction() method:

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// src/AppBundle/Controller/DefaultController.php
namespace AppBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\Routing\Annotation\Route;

class DefaultController extends Controller
{
    /**
     * @Route("/", name="homepage")
     */
    public function indexAction()
    {
        return $this->render('default/index.html.twig', [
            // ...
        ]);
    }
}

These three lines define the routing configuration via the @Route() annotation. A PHP annotation is a convenient way to configure a method without having to write regular PHP code. Beware that annotation blocks start with /**, whereas regular PHP comments start with /*.

The first value of @Route() defines the URL that will trigger the execution of the action. As you don't have to add the host of your application to the URL (e.g. http://example.com), these URLs are always relative and they are usually called paths. In this case, the / path refers to the application homepage. The second value of @Route() (e.g. name="homepage") is optional and sets the name of this route. For now this name is not needed, but later it'll be useful for linking pages.

Considering all this, the @Route("/", name="homepage") annotation creates a new route called homepage which makes Symfony execute the index action of the Default controller when the user browses the / path of the application.

Templates

The only content of the index action is this PHP instruction:

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return $this->render('default/index.html.twig', [
    // ...
]);

The $this->render() method is a convenient shortcut to render a template. Symfony provides some useful shortcuts to any controller extending from the base Symfony Controller class.

By default, application templates are stored in the app/Resources/views/ directory. Therefore, the default/index.html.twig template corresponds to the app/Resources/views/default/index.html.twig. Open that file and you'll see the following code:

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{# app/Resources/views/default/index.html.twig #}
{% extends 'base.html.twig' %}

{% block body %}
    <h1>Welcome to Symfony</h1>

    {# ... #}
{% endblock %}

This template is created with Twig, a template engine created for modern PHP applications. The second part of this tutorial explains how templates work in Symfony.

What is an Environment?

An environment represents a group of configurations that's used to run your application. Symfony defines two environments by default: dev (suited for when developing the application locally) and prod (optimized for when executing the application on production).

When you visit the http://localhost:8000 URL in your browser, you're executing your Symfony application in the dev environment. To visit your application in the prod environment, visit the http://localhost:8000/app.php URL instead. If you prefer to always show the dev environment in the URL, you can visit http://localhost:8000/app_dev.php URL.

The main difference between environments is that dev is optimized to provide lots of information to the developer, which means worse application performance. Meanwhile, prod is optimized to get the best performance, which means that debug information is disabled, as well as the web debug toolbar.

The other difference between environments is the configuration options used to execute the application. When you access the dev environment, Symfony loads the app/config/config_dev.yml configuration file. When you access the prod environment, Symfony loads app/config/config_prod.yml file.

Typically, the environments share a large amount of configuration options. For that reason, you put your common configuration in config.yml and override the specific configuration file for each environment where necessary:

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

web_profiler:
    toolbar: true
    intercept_redirects: false

In this example, the config_dev.yml configuration file imports the common config.yml file and then overrides any existing web debug toolbar configuration with its own options.

For more details on environments, see the "Environments" section of the Configuration guide.