title: How to track users cross domain for a purchase
body: <p>I'm creating a kind of affiliate marketing website with products I've scraped from partner websites. The user clicks on the Buy link and is re-directed to the partner site to complete the purchase. If the user buys the product, I should get a percentage of the sales. So far this model is a pretty common approach.</p>
  
  <p>However the element that I don't yet understand is, what technology or script do other affiliate sites use to track to see if the user, once they've let the main site, does purchase the product or not? How do I track my user's purchase or actions once they've left my site?</p>
  
  <p>I know that cookies can store brief user information but I don't think that's enough. I need to then store this information in a database. Should I use php sessions for this?</p>
  
  <p>Any tips or suggestions is really appreciated. 
  Cheers</p>