function getIDs() {
    var x = <?php if(isset($_SESSION['IDs'])) echo $_SESSION['IDs']; ?>;
    if(x == '') {
        $.post('getPL.php','PL_name=cool&q=IDs',function(responseText){
            x = responseText;
        });
    }
    //some other code
    //return array; // this is an array i use later in js
}

$videos_list = $theOne->parentNode->parentNode->getElementsByTagName('video');
    for ($i = 0; $i < $videos_list->length; $i++) {
        $a = $videos_list->item($i);
        $id_out = $a->getElementsByTagName('id')->item(0)->nodeValue;
        $array[$i] = $id_out;
    }
    $IDs = json_encode($array);
    $_SESSION['IDs'] = $IDs;
    echo $IDs;
    break;