I am using a login form on Symfony2 with the following controller code
public function loginAction(Request $request)
{
$user = new SiteUser();
$form
If you're using Symfony 2 security management, you don't need to get posted values, you only need to manage form template (see documentation).
If you aren't using Symfony 2 security management, I advise you strongly to use it. If you don't want to or if you can't, can you give us the LoginType's sources ?