I need a paypal order processing IPN script with the following features:
I will be selling serial numbers on my site.
It must be hacker proof, so if someone posts fake paypal details it shouldnt accept that.
It must check payment amount, depending on the paid amount 3 type of serial numbers are chosen from a mySQL database. eg: if a user paid $1 he gets serial from table 1. if he paid $2 he gets serial from table 2. if he paid $3 then he gets serial from table 3.
It should also show the user a thank you page with his serial number in it.
In case of a payment failure it needs to show the user a payment failure page.
Please note that it is very important that the script should be cheater-proof.