Depleting a database quantity with online shopping cart/order form

[cineplex]

Basically, I've got an online order form that's dynamically generated based on the contents of an inventory database. The form should allow the user to purchase items from the form which will then be deleted from the database.

Each product is on a line in the form. Each line has a list box from which the user can choose a quantity to purchase.

QUESTION

I need to get the quantity that the user selected from the list box on the form and subtract it from the quantity of the correct item in the database.

What'd be a good approach to this?

Thanks!!

[TTT]

i assume that you have

item --- amount --- button
=> solution
<javascript>
function addthis(the_id) {
document.formi.yourselection.value = the_id
document.formi.submit();
}
</javascript>

item ---
input type="text" name="amount$id" ---
input type="button" onclick="javascript:addThis($id)"
input type="hidden" name="yourselection"

=> submit
=> you get yourselection = id
$selected_amount = "amount".$yourselection;
==> you can put value in session
$HTTP_SESSION_VARS['amount'] = $$selected_amount;