Did you try googling for "php shopping cart"?! Try that and you will be
amazed at how many hits you will get.
I have tried phpshop and found it okay (too difficult to customize,
IMO). There is also another called fishcart but something with the
license turned me away (that was a long time back, so can't really
pinpoint the reason).
So, I ended up writing my own. If you are using Paypal, Paypal API
pretty much takes care of the shopping cart aspect too!
If you want to write something from scratch, may be you can start off
reading the 3 part tutorial series at zend.com:
http://www.zend.com/zend/spotlight/p...ping-cart1.php
Thanks,
--Kartic