Question
I am creating a php/mysql application for a university project and I am looking for any possible solution to allow users to buy from my site .
Solution
PayPal has a number of solutions for you, including services for Payflow Pro.
OTHER TIPS
We use Saferpay via command line utility. Will gladly follow this post for better options.
Are you planning on writing the shopping cart as well?
We have been evaluating the Magento E-Commerce Platform, our initial tests show that it is pretty robust. It supports a bunch of payment gateway options.
Check it out... http://www.magentocommerce.com/
An unofficial implementation in PHP: https://github.com/payment/saferpay
Protocol is described in https://www.six-payment-services.com/dam/saferpay/Saferpay_Payment_Page_EN.pdf, chapter 4.