Error while using PDO prepared statements and LIMIT in query

I'm using PDO in my application. But I have a problem while I'm working with prepared statements in a query that contains LIMIT. What's the problem?
Codes:

$start = 0;
$rows = 20;
$sql = "SELECT * FROM tbl_news ORDER BY date DESC LIMIT ?, ?";
$q = $db->prepare($sql);
$q->execute(array($start , $rows));

Error:

check the manual that corresponds to your MySQL server version for the right syntax to use near ''0', '20''