110335 Affiliate Program Needs Cookie

I operate an established web site that has its own Affiliate Program. It is a PHP based site with a MySQL database. Currently the Affiliate Program uses PHP Session Codes to track users. But the Session Codes expire when the user leaves the site. I would like to add cookies so that visitors can return to the site later and the original affiliate still gets credit for the sale. I am looking for someone who can add cookies to my existing site. Or if there is a better way to do this, let me know. The important thing is that the affiliate gets credit for a sale even if the visitor returns to my web site three days after they first entered using the affiliate's URL code. For your reference, each affiliate gets a link code like this: [login to view URL] Post a message to the message board if you need further information from me. Thanks.