Changing Webhosting Signup Form

I have a webhosting company, that needs its signup form done in PHP to be slightly changed. Would like extra field to be added in that allow bonus codes to be entered in by customers, and upon being validated, the customer is sent to the correct payment page. ## Deliverables Essentially, we have released some bonus codes via 2checkout (Our credit card processor), only to find it only applies for the first month. Paypal advised they dont do the discount codes for merchants. What I essentially want, is for customers to be able to put the codes in when signing up, and it will automatically see that if code "HELLO" is put in, and matches with plan "BYEBYE", and payment method is "PAYPAL", then follow special link that is at the lower price, instead of normal link. The signup form is already automated, its just the extra checks in place I require for bonus codes. I would like also some documentation so I know how to change these later on, and if I want to add any, etc. Thanks