Simple Secure PHP IPN script needed

We have a site with two different things for sale (Owned License and Leased License for a script), they are purchased through "Buy Now" buttons. We need a simple and secure IPN script that will just check if the price is correct and if other things are correct and then execute some code that we have to set the user to paid and set other variables (based on if they bought Leased or Owned License).

This should be a simple job for someone who knows how to use PHP IPN scripts, only post if you have experience with PHP IPN scripts from PayPal.

I will pay through PayPal

Why wouldn't you just be able to use http://www.dinkumsoft.com/dsd/ipnphp.php