Doctrine DQL execute passing params

Question

I used this DQL in Doctrine

$q->update('product')
->set('quantity','?')
->where('id=?');
$q->execute(array(20,5));

I check the server for the query and this the generated sql

    UPDATE product SET quantity = '20', updated_at = '5' 
WHERE (id = '2010-04-26 14:34); 

So I need to know why the arguments aren't in the correct places?