hummm, i smell a rat.
i think you're suggesting that you can hack at other peoples code and get it to work, but cant make your own - is that right? Nothing wrong with that, i hack at other people's stuff a lot myself. i would say though that you need a book on the matter (personal opinion, i learn better from books), and I use PHP and MySQL Web development by luke welling and laura someone or other.
The PHP book i reccommend has good tutorials (although a few mistakes, but they're fairly easy to spot), sample code, and chapters dedicated to projects like content management, making your own shopping cart etc. I'd advise getting it and working through it, keeping in mind what your project is.
You may also want to check out php builder , but they can be fairly reluctant to help sometimes. And I can pretty much guarantee that if you post a question like you have above then you'll get very little help at all. I think you'll find that people will be more willing to help with actual code, and they (myself included) like to see that people have had a go themselves. It's always quite frustrating when someone comes along and says something like "I'm trying to make an online shopping system and i cant get it to work - so far i've got: <html></html>. any idea where i go next?", since that shows that they're not prepared to look around and do some research themselves, they just want the answer in their lap! Get a good book, and once you've used it to learn the basics you'll have a good idea of how what you want done should be tackled. Then you can post questions about particular elements of code - cos if your learning is anything like mine, you'll have a lot of problems like that!
Good luck with it - I've really enjoyed learning PHP (I only started at christmas and i'm already designing an intranet for a company i know for a pretty fee!) and still have a lot to learn, but you have to work at it yourself, otherwise you never quite understand where you were going wrong in the first place. And if you'd like to share intranet ideas sometime, that'd be cool too!