$freeID=(int)$db->get_var('select min(Message_ID)+1 mid from Message'.$classID.' t1 where not exists (select 1 from Message'.$classID.' where Message_ID=t1.Message_ID+1)');
if ($freeID>0 && $msgID!=$freeID) {
  $db->query('update Message'.$classID.' set Message_ID='.$freeID.' where Message_ID='.$msgID);
}