What you want to do is quite a big project, if you don't know a huge amount about php. I would suggest looking at http://www.zend.com/zend/tut/authentication.php, it's a good tutorial. You'll need to be using http headers, and session management, and each page will need to check if you're logged in, usually by checking a session variable. Anyway, check out the site, and good luck! 🙂
