Using YAML:

# app/config/routing.yml
blog_list:
    path:     /blog
    defaults: { _controller: AppBundle:Blog:list }

Using Annotations:

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

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;

class BlogController extends Controller
{
    /**
     * @Route("/blog", name="blog_list")
     */
    public function listAction()
    {
        // ...
    }
}

A request for the /blog URL will be handled by the listAction() method of the BlogController inside AppBundle.

Using YAML:

# app/config/routing.yml
blog_show:
    path:     /blog/{slug}
    defaults: { _controller: AppBundle:Blog:show }

Using Annotations:

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

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;

class BlogController extends Controller
{
    /**
     * @Route("/blog/{slug}", name="blog_show")
     */
    public function showAction($slug)
    {
        // ...
    }
}

Any request with a URL matching /blog/* will be handled by the showAction() method of the BlogController within AppBundle. The controller action will receive the value of the placeholder as a method argument.

For example, a request for /blog/my-post will trigger a call to showAction() with an argument $slug containing the value my-post. Using that argument, the controller action can change the response depending on the value of the placeholder, for instance by retrieving the blog post with the slug my-post from the database.

If you want to have a placeholder that may be omitted, you can give it a default value:

Using YAML:

# app/config/routing.yml
blog_list:
    path:      /blog/{page}
    defaults:  { _controller: AppBundle:Blog:list, page: 1 }
    requirements:
        page: '\d+'

Using Annotations:

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

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;

class BlogController extends Controller
{
    /**
     * @Route("/blog/{page}", name="blog_list", requirements={"page": "\d+"})
     */
    public function listAction($page = 1)
    {
        // ...
    }
}

In this example, both the /blog and /blog/1 URLs will match the blog_list route and will be handled by the listAction() method. In the case of /blog, listAction() will still receive the $page argument, with the default value 1.