title: PHP PayPal payment validation
body: <p>I'm developing a site for a client, and he wants people to be able to buy licenses/support contracts via PayPal. What I'm stumped on is how I can make the payments secure. My current idea is to generate a random string using MD5 or whatever, plug this into a database, and send it along with the rest of the PayPal button code. When the payment is completed, the user is redirected to a PHP page where there is code to update their information in the database. The only thing stopping a hacker giving themselves a license/whatever without paying is the random hash, which can be found out using a sniffer of some sort, and plugged into the POST data returned by the PayPal code.</p>
  
  <p>So. My question is this: How do I execute custom PHP code <em>only</em> on a successful PayPal payment, without leaving any loopholes open for the more evil users of the interwebs?</p>