HOW TO: Reload a frame with a different URL?

Hello everyone, Sorry for the simple question but I'm trying to learn a little more about PHP and just need a little help. I'm currently running PHP3 if that helps. I just need to reload the "main" frame on my site with a new page once the user has logged in. All the navigation and login are handled from a nav frame with PHP script that checks if the user is authenticated. I know I need to put the code in my if...else loop but I'm not sure what the code is. Thanks for any help!! MarcJ