113489 Paypal IPN notify checker

I need a program that will work with paypal's IPN system to check for payment status, if the IPN is legit, and to clearly marked place in the coding where I can place PHP and MySQL code to do what I need depending on status of payment. I also need it to be able to get the payment status if it changes to pending to complete/fail at time of change. I may be using this system for anything that is just real symple on product payment to a very complex multi-cart system. I need this program to as universal as possible and to check for any possable fraud in accepting payment.