I have the value "amount" defined by an invisible field as "16".

I need to have it so that if a customer chooses the second option, "within 7 days", the value of "amount" will increase by 10.

Here is what I have so far:

<select name="amount" id="amount">
                <option value="???">Guaranteed within 30 days</option>
                <option value="???">Guaranteed within 7 days (add $1 CPM)</option>
              </select>

I know I need to put something in place of the ???'s, but not sure what. The first one needs to say "Leave the amount as it is", and the second one needs to say, "Add 10 to the amount".

Any help would be greatly appreciated.

Surely if the amount is always 16, you'd simply have 26 in the second select option, and 16 in the first.... you're questions does not make much sense and has little, if any, relevance to PHP.

However, if you mean you want to dynamically create this, then the mathematical functions can be found in the PHP Manual

Ah. Wrong forum, then. Apologies.

I can't have the second value as 26, but need to add 10 instead. This is because underneath I will need another menu that will have the same/similar effect.

leavethisplace is quite right, so far you have not explained any valid reason for not simply entering '16' and '26' as values. "underneath I will need another menu that will have the same/similar effect", doesn't make any sense as to why you need the form constructed this way.

I need it constructed in this way so that the customer can choose multiple options, so they can use two different drop menus to change the values.

For example, if they can then choose, on thje first menu, to add 10 to the amount, and on the second, add 5.

Thanks for the help.

Oh, and could a moderator please move this post?