Need Javascript Tracker (similar to Google Analytics, but simpler)

Hello, I need a script built that does some very basic tracking. Here's the situation - I have a partnership with a site where I send them leads. They pay me a percentage of the final sale. I need some generic javascript that they can install that allows me to track the conversions in order to keep them honest. So, user follows my link... it tracks that user, and if they complete the purchase, it notifies my server. It's pretty similar to what Google analytics does: <[login to view URL]> On the server side, PHP is preferred, but make suggestions as to how this would work/communicate. Thanks, Dan