Integrate Paypal with Website

Hi, I've got a fully working custom-made shopping cart that I've made myself with PHP and MySQL. I'm trying to integrate paypal with it but I'm having some problems. The cart can send data to paypal but I am not sure about how best to receive the data the Paypal sends back and how to make it work properly. I currently already use Worldpay with the website, I have coded this without any problems. I need a coder to help me finish integrating paypal with the website. This should be a very simple job for most experts. The code already works fine with Worldpay, you just need to copy and edit it to accept Paypal's payments. I need this job done as soon as possible. Thanks.