If you need a shopping cart because you want to sell something online, there are all kinds of open source (free) shopping carts that you can download and use. If you use PayPal you can even use their shopping cart. If you are doing it as a learning exercise, then you are probably still better off to download one and then rip it apart to figure how it works (and make it better if you have the time and interest. There are lots of sources. You could start with hotscripts.com.

Yes it is always better to see some of the existing open source php shopping cart scripts like for e.g opencart etc and learn from them.

If you just practice coding PHP shopping cart, find some ebooks to see the basic of shopping cart, it will be easy for you to start. If you want to use shopping cart for online shopping, it is better if you use opensource shopping cart.

you can use zencart. its good option of simple shopping cart.

you can download from here. its very easy to integrate.

url is http://www.zen-cart.com/

Whatever you do, don't use Zen Cart.
It uses insane numbers of queries (tens to hundreds, literally) for pages where 5-10 queries would do the job just as good or better.

Opencart is a lot more sensible (at least it was when I've tried it out the last time, which was before v. 1.0, although they've bumped up the number quite a bit with the last few release(s)).