I agree with McGruff and JPlush (hey, for once, J

). If you are a newbie to web-technolgies and PHP, the issues are simply too complex.
For step-by-step tutorial to shopping carts I would recommend "MySQL/PHP Database Applications" by Jay Greenspan and Brad Bulger.
Write an online shop that handles orders via email, i.e. once an order is placed an email gets sent to the supplier/owner of the shop.
Don't ask for credit card details. Let the supplier/owner handle that on receipt of that order-email.
All you'll be processing is product details - no payment methods or anything else.
So strictly speaking: start off with an online ordering application. That will give you enough to do for a bit and you'll learn much more about PHP and web technologies.
However, do
not even dream about integrating credit cards into that ordering system. That would be like asking around for a third party to grab the card details.