Php Affiliate Marketing Script Modifications

Hello Programmers, How are you doing? I need help modifying my affiliate script. Currently, it works just like any other affiliate script out there. Affiliates sign up, the can promote the site with text links/banners that contain their affiliate ID/url, and they are given credit (a commission %) for any item that is sold. This all works great! I would like to modify the script, so that affiliates can also choose a word or a phrase to associate with their affiliate account as well. If this phrase or word is entered at checkout, then the affiliate will receive credit for the sale. Each affiliate will need to choose a unique/custom phrase. They will all be different/no duplicates. (This would be similar to how users can choose their own url / on sites such as myspace/facebook.) I would like to get started working on this ASAP. Thanks, BCBC