piecing together basic knowledge of paypal nvp api & PHP - to work together

Hi everyone I have to produce just now, quickly, and so bridge 2 things I still know very little about (PHP and paypal), so I'll have to show my ignorance here. Anyone who has experience with the paypal nvp api, maybe you can help me?: I am successfully using the sample scripts paypal provides, but moving towards our own solution, I am making a link (the paypal express checkout button) from our shopping cart page to another page on our site where we do stuff and then redirect to the nvp api. I want to stick the payment amount (and other things) in that link's parameters, but then it is hackable (i.e. bad). My first thought is that i need to read about PHP sessions (which I have never done), so I can store stuff that I need to associate with the user's cart, but have it from the user, but then I thought I should start here to see if anyone can just tell me what you have done with this. I have so much homework to do I would like to start with what will actually solve my issues now rather than read the whole library just to complete this assignment. Thanks, -Govinda