DemoController.php

<?php

namespace Acme\DemoBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

use Symfony\Component\Security\Core\Role\SwitchUserRole;

use FOS\UserBundle\Propel\UserQuery;

class DemoController extends Controller
{
    public function yourAction(Request $request)
    {
        $parameters = array();

        // plenty of stuff to do?

        $security = $this->get('security.context');
        if ($security->isGranted('ROLE_ALLOWED_TO_SWITCH') or $security->isGranted('ROLE_PREVIOUS_ADMIN')) {
            $parameters['users'] = UserQuery::create()->find();
            $parameters['original_user'] = $this->get('security.context')->getToken();

            foreach ($security->getToken()->getRoles() as $role) {
                if ($role instanceof SwitchUserRole) {
                    $parameters['original_user'] = $role->getSource();
                    break;
                }
            }
        }

        return $this->render('AcmeDemoBundle:Demo:yourAction.html.twig', $parameters);
    }
}

example.png

yourAction.html.twig

<!-- twitter bootstrap snippet -->

<p class="pull-right">Logged in as <span>{{ app.security.token.username }}</span></p>
{% if users is defined %}
    <ul class="nav secondary-nav">
        <li class="dropdown" data-dropdown="dropdown">
            <a href="#" class="dropdown-toggle">Switch User</a>
            <ul class="dropdown-menu">
                {% for user in users %}
                {% if user.username != original_user.username %}
                    <li><a href="?_switch_user={{ user.username }}">{{ user.username }}</a></li>
                {% endif %}
                {% endfor %}
                <li class="divider"></li>
                <li><a href="?_switch_user=_exit">{{ original_user.username }}</a></li>
            </ul>
        </li>
    </ul>
{% endif %}