facebook php sdk: retrieve user information

I have a problem with retrieve basic user information. This is my code:

require_once 'facebook-php-sdk/autoload.php';
FacebookSession::setDefaultApplication('appid', 'appsecret');

try {
    $token = 'mytoken';
    $session = new FacebookSession($token);
    $request = (new FacebookRequest($session, 'GET', '/me'))->execute()->getGraphObject(GraphUser::className());
     $username = $request->getName();

    echo $username . '<br>';
} catch (FacebookRequestException $e) {
    echo'Error';
    echo $e;
}

but no exception is throws, and nothing is printed. What's wrong?