1_security.yml

    //you may have some more configs here...
    providers:
        user:
            entity: { class: MyAppBundle:User, property: loginEmail }
        admin:
            entity: { class: MyAppBundle:Admin, property: username }
    firewalls:
        dev:
            pattern:  ^/(_(profiler|wdt)|css|images|js)/
            security: false
        login:
            pattern:  /login$
            security: false
        frontend:
            provider: user
            anonymous: ~
            pattern: ^/user
            form_login:
                login_path: /user/login
                check_path: /user/login_check
            logout:
                path: /user/logout
                target: /user/
        backend:
            provider: admin
            pattern: ^/admin
            form_login:
                login_path: /admin/login
                check_path: /admin/login_check
            logout:
                path: /admin/logout
                target: /admin/
            #anonymous: ~
    access_control:
        - { path: ^/user/switch$, roles: IS_AUTHENTICATED_ANONYMOUSLY }
        - { path: ^/user, roles: ROLE_USER }
        - { path: ^/admin, roles: ROLE_ADMIN }

2_routing.yml

//you may have some more routings here...
admin_switch_to_user:
    pattern: /admin/user/switch/{userId}
    defaults: { _controller: MyAppBundle:Admin\User:switch }
    requirements: { userId: integer }
user_switch:
    pattern: /user/switch
    defaults: { _controller: MyAppBundle:Account:switch }
user_switch_exit:
    pattern: /user/exit_switch
    defaults: { _controller: MyAppBundle:Account:exitSwitch }

3_AccountController.php

use Symfony\Component\Security\Core\Authentication\UsernamePasswordToken;
use Symfony\Component\Security\Http\Event\InteractiveLoginEvent;

//you may have some more action methods here...

    public function switchAction()
    {
        if ($this->get('session')->get('userid_to_switch'))
        {
            $user = $this->getDoctrine()->getEntityManager()->find('MyAppBundle:User', $this->get('session')->get('userid_to_switch'));
            if ($user)
            {
                //create token instance
                //2nd argument is password, but empty string is accepted
                //3rd argument is "firewall" name(be careful, not a "provider" name!!! though UsernamePasswordToken.php names it as "providerKey")
                $token = new UsernamePasswordToken($user, '', 'frontend', $user->getRoles());
                //set token instance to security context
                $this->get('security.context')->setToken($token);

                //fire a login event
                $event = new InteractiveLoginEvent($this->getRequest(), $token);
                $this->get('event_dispatcher')->dispatch('security.interactive_login', $event);

                //redirect to some user scoped actions
                return $this->redirect($this->generateUrl('user_homepage'));
            }
        }

        //fallback
        return $this->redirect($this->generateUrl('admin_homepage'));
    }

    public function exitSwitchAction()
    {
        //unset "userid_to_switch" in session to avoid conflicts
        $this->get('session')->set('userid_to_switch', null);
        //logout as user
        $this->get('security.context')->setToken(null);

        //going back to admin context...
        return $this->redirect($this->generateUrl('admin_homepage'));
    }

4_Admin\UserController.php

//you may have some more action methods here...

    public function switchAction($userId)
    {
        //never use the session key "userid_to_switch" in other actions
        $this->get('session')->set('userid_to_switch', $userId);
        
        return $this->redirect($this->generateUrl('user_switch'));
    }

5_users_base.html.twig

//how to render a link to "exit_switch" in switched user context

{% if app.user %}
    {% if app.session.get('userid_to_switch') %}
        <a href="{{ path('user_switch_exit') }}">{{ 'Re-switch as admin'|trans }}</a>
    {% else %}
        <a href="{{ path('user_logout') }}">{{ 'Logout'|trans }}</a>
    {% endif %}
{% endif %}