isLiked（）在PHP的phpsdk4中没有返回任何内容

I have this problem. I was created facebook tab app application in developer fb, app I'm turn on.

I use this code. Unfortunately it doesn't work function isLiked(). Where is problem?

<?php
require 'phpsdk4/autoload.php';

use Facebook\FacebookSession;
use Facebook\FacebookPageTabHelper;

FacebookSession::setDefaultApplication('XXX', 'YYY');

$helper = new FacebookPageTabHelper();

echo '<p>page id: ' . $helper->getPageId() . '</p>';
echo '<p>liked: ' . $helper->isLiked() . '</p>';  
echo '<p>admin: ' . $helper->isAdmin() . '</p>';
?>

Function pageid and admin work correct. Unfortunately function isLiked() return nothing. Where is problem?

Thank you

Edit: I am try refresh page, if page like or not, however function return nothing.