Forum Moderators: coopster
Have you had a look at some of the open source code at sourceforge, freshmeat, hotscripts, etc? You're likely to find something there that you could start with. Also, don't forget to browse the WebmasterWorld Ecommerce Forum [webmasterworld.com].
My first foray into PHP was customizing an existing shopping cart. I wasted a lot of time working on one cart that had just terrible spaghetti code [webmasterworld.com] - it would literally open a tablein one file and close it in a file that was included by a file that was included in the original file.
So don't get too invested in any particular cart until you find one that has a general architecture that seems organized and makes sense to you. You'll be way better off IMO by putting some time in up front to review the code in several packages than by committing to one too early. If you have to look at four or five before you find one that's right for you, don't despair.