Add cookies tracking script to existing session tracking script

I have a tracking script that uses a php session to give credit for referring a person to our site, right now when you close the browser the session is lost. I want to add a cookie so the session will be saved for at least 1 week. You will have to add this to the current script without altering the way the old script works. This is so that we do not have any problems with existing members. Im also looking for a part time freelancer to work on other projects. So if I like you work on this project then I will consider hiring you.