156320 Modification To PHP Script

I have a feature on a job website that allows me to create a "promo code" for advertising to new clients. The promo code can be set to offer as many free ad listing credits to new clients that I choose. It works great, the only problem is the promo code is blanket. Meaning, if I designate a promo code to offer 1 credit, it can be only used one time. If I set it to offer 10 credits, it will load 10 credits for the first person who uses it and will then go "inactive". What I need is for one of you fine folks to go into the existing code and simply modify it to work like this: If I create a promo code and give it 1 credit, that same promo code will be able to be used by multiple usernames. Simply put, the credit will be attached to a username, or client email to be used ONCE by a single client. Should be pretty easy for someone who knows PHP