In the navigation tutorial, they show you a simpel way to use the switch() control to display different content based on a variable set in the $_GET array (URL string).

The second tutorial, Single-Page Password Protection, shows you quickly how to create a form that takes in a password and checks it against one hard-coded into the PHP script. If it validates, the "secret content" is shown.

Both of these tutorials are very simple, and, if you're not careful with them, could cause some security issues with your scripts. All in all, though, they're a good starting place...