Automating PayPal Payments via PHP?

I have a software that has serial keys required to activate it. I want to be able to let users purchase my so开发者_开发问答ftware via PayPal then automatically have the serial keys sent to them via email. I want to integrate it to PHP for more flexibility but I can't seem to find any good tutorials on how to do it?

If anyone can help, it would be much appreciated!

There are several options in paypal. If you use the simplest buy now button, all you have to do is make your ipn page send them the email with the serial key. Basically the posted information can be accessed in the ipn page. There are other ways to do it but this is the simplest one.