A simple code for a form login by firewall in FOSUserBundle (Symfony2). One concept : overwriting.

gistfile1.php

<?php

namespace Pr3ss\UserBundle\Controller;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\Security\Core\SecurityContextInterface;
use Symfony\Component\Security\Core\Exception\AuthenticationException;
use FOS\UserBundle\Controller\SecurityController as BaseController;

class SecurityController extends BaseController {
    protected function renderLogin(array $data) {       
        // get target path
        if(strpos($this->container->get('request')->get('_route'), 'backend') !== false) {
            $template = 'Pr3ssUserBundle:Backend/Security:signin.html.twig';
        } else {
            $template = 'Pr3ssUserBundle:Frontend/Security:signin.html.twig';
        }

        return $this->container->get('templating')->renderResponse($template, $data);
    }
}

gistfile2.yml

# routing.yml
backend_login:
    pattern:  /backend/login
    defaults: { _controller: FOSUserBundle:Security:login }

backend_login_check:
    pattern:  /backend/login_check
    defaults: { _controller: FOSUserBundle:Security:check }

backend_logout:
    pattern:  /backend/logout
    defaults: { _controller: FOSUserBundle:Security:logout }