MySQLi更新DATETIME列

I know this is going to be something simple, but it's driving me a little insane at the moment. I need to update my datetime column with my statement, however it's not working. Any ideas?

$stmt = $db->prepare("UPDATE clients SET client_last_date = NOW() WHERE client_id = ?"))
$stmt->bind_param("i", $_SESSION['client_id']);
$stmt->execute();
$stmt->close();

The error i'm getting is Fatal error: Call to undefined method PDOStatement::bind_param()

EDIT - Turns out there was conflicting PDO objects inside a login script im using, that was causing my error. All sorted now!

I think it's written otherwise in PDO:

$stmt->bind_param("i", $_SESSION['client_id']);

should be

$stmt->bindParam(1, $_SESSION['client_id']);

in mysqli it would be bind_param but the error speaks of PDO. $db must be a PDO ressource...

mysqli http://ca.php.net/manual/en/mysqli-stmt.bind-param.php

pdo http://www.php.net/manual/en/pdostatement.bindparam.php

I don't know what Database you are using...

It might be something like :

"UPDATE clients SET client_last_date = (select sysdate from dual) WHERE client_id = ?"