Help needed

i have a little problem which is

1. i have table called payment with the following text fields
   
   -----Cost1
   
   -----Cost2
   
   -----Cost3
   
   ----- Amount total {it calculates the cost fields and store the total amount here}
   
   ------Amount Paid {}
   
   ------Balance { it subtract the amount paid from the amount total and store the balance here}
   My problem is this hw to insert php code to do this calculation using the php runner and also to make sure the value entered in amount paid field is not greater than amount total. Am using the PHP RUNNER 5.2