This could be the wrong way to do it, but I've had success with the following code -
$dateUF=strtotime($submittedDate);
$dateF=date("Y-m-d", $dateUF);
This'll take the submitted date field, convert it to a timestamp ($dateUF), then convert the timestamp to the MySQL format date ($dateF). Works to and from the database.
jay_suz - simply concantenate the fields, like so -
$dateF = $year."-".$month."-".$day;
You'll also need to make sure that any day or month specification includes the preceding 0, but I can't remember off the top of my head how to take car of that one. sorry...
HTH!