A secure Paypal payment in PHP

Dear all, I have a PHP form that makes a payment on Paypal. The form is attached. The problem with this is that the buyer can edit the form and change the price to a lower one. I need a more secure way of sending a payment to Paypal. A PHP function would be just fine as it would allow me to send the payment from the server and not from the client browser, thus avoiding the form data tampering. How to go about all of this..? Because it is for a web shop in a CMS, a server side solution would be prefered over an encrypted form if there is such a possibility. ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Deliverables must be in ready-to-run condition, as follows (depending on the nature of the deliverables): a) For web sites or other server-side deliverables intended to only ever exist in one place in the Buyer's environment--Deliverables must be installed by the Seller in ready-to-run condition in the Buyer's environment. b) For all others including desktop software or software the buyer intends to distribute: A software installation package that will install the software in ready-to-run condition on the platform(s) specified in this bid request. 3) All deliverables will be considered "work made for hire" under U.S. Copyright law. Buyer will receive exclusive and complete copyrights to all work purchased. (No GPL, GNU, 3rd party components, etc. unless all copyright ramifications are explained AND AGREED TO by the buyer on the site per the coder's Seller Legal Agreement). ## Platform PHP 4.3.11