PHP PayPal payment verification / HTTP POST?

I am working on a software project and have written a validation system to help prevent theft of the software. I was reading back through the code, and found a huge "loophole" that could potentially allow the motiviated users to copy the program and install it without even contacting the activation server. So, I modified the program and the server to patch that up. Now, I need a PHP page that can check to make sure the user paid for the software, payments made using PayPal, then generate a random string. I then need the page to POST that random string to the validation server ( HTTP ) a开发者_如何学编程nd listen for the response from the server. It will need to POST this parameter: Validator = [Random String] It will tell the page if the random string has already been submitted and to try another, or that it accepted the string. I would like the string to be numerical, but I think alpha - numeric would be better and give more options. I am pretty confident this can be done, but am not sure how to do the PHP part.

Which PHP part?

You could generate a random string with md5.

To POST from a PHP script, just use curl.