This has been racking my brain all day, I just can't get this update to work....
UPDATE mKart
SET krtUsr = '$UsersID',
SET KrtProd = $ProdId,
SET KrtNum = KrtNum + 1
WHERE
krtUsr = '$UsersID'
AND
KrtProd = $ProdId
Please note, this message has been edited for seciruty, however, the same problem is shown here... just edited the vars.