Auto-Post form data

Hi all, I've got the following problem: I want to offer a 'Buy now' button on my web page that POSTs a form to a site, that I do not have control of, which processes payment. The form data must contain the price of the item as a hidden element. Thus a fraudster could use the source of my page and change the price. As I want to redirect the customer who payed correctly to a download site, there's no more interaction from my side, so I can't check manually, if the price is still OK. Therefore I thought it would be a solution to first post the data to a PHP page on my site, which checks all data and then automatically reposts form data to the site that processes payment (or rejects the request, if something's wrong). Unfortunately I don't know, if and how it's possible to post form data from a PHP script without any more user interaction. Thanks for any help! Regards, Gerhard Petrowitsch P.S.: If my description was unclear, here's a summary HTML form --> posted on user reqest to PHP script --> checks data ---> posts automatically to external site Is the second automatic post possible? How?