Getting the user id from logged in user in FOSUserBundle

You don't need to create new user object $user = new User(); ,you can the current logged in user object from security context

$user = $this->container->get('security.token_storage')->getToken()->getUser();
$user->getId();

Edit: security.context is deprecated, use security.token_storage instead. For more details : https://symfony.com/blog/new-in-symfony-2-6-security-component-improvements

simply writing

$user = $this->getUser()->getId();

You get the Id of the current user

This is working for me with Symfony 3

if( $this->container->get( 'security.authorization_checker' )->isGranted( 'IS_AUTHENTICATED_FULLY' ) )
{
    $user = $this->container->get('security.token_storage')->getToken()->getUser();
    $username = $user->getUsername();
}