Update form objects before submit

I have a html page including php scripts that perform different stuff. The page contains a form (a Select 'products-price' and a field 'qty'). Depending the choice made by the user, I need to know what item in the SELECT object was choose and the number entered in the 'Qty' field to initialized a 'subtotal' zone BEFORE the user submit the form. This functionality is common to all e-business sites when you have to clic on the 'update' button before submit the form. Any idea ? thanks ---------------------------------- Stéphane Pinel 39, Rue du Docteur Heulin 75017 Paris (FRANCE) 33 1 53 11 05 77 33 6 08 94 63 16 spinel@noos.fr ----------------------------------