Kaltura PHP API v 3 filter by startDate issue

kaltura_filter.php

<?php

//....

//sample code
$cat_id = '8317721';
$page_id=1;
$pageSize=15;

$pager = new \Kaltura\Client\Type\FilterPager();
$pager->pageIndex = $page_id;
$pager->pageSize = $pageSize;

$filter2 = new \Kaltura\Client\Type\MediaEntryFilter();
$filter2->orderBy = Kaltura\Client\Enum\MediaEntryOrderBy::START_DATE_DESC;
/*
have tried :
$filter2->orderBy = 'startDate';
$filter2->orderBy = '-startDate';
$filter2->orderBy = 'START_DATE'
*/
$filter2->categoriesIdsMatchAnd = $cat_id;

$entries = $client->media->listAction($filter2, $pager);

//....

?>