Facebook使用新的php SDK获取页面管理员

I have made a code for getting page id, page admin but I only get the Page Id. My code is:

<?php
session_start();
require_once 'admin/facebook/autoload.php';

use Facebook\FacebookSession;
use Facebook\FacebookPageTabHelper;

FacebookSession::setDefaultApplication('app_id', 'app_secret');
$pageHelper = new FacebookPageTabHelper();

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

What am I doing wrong? Thank you