Understanding MVC

I am in the process of creating a website from scratch, which is proving to be my first real encounter with PHP. For a while, I was looking at several PHP frameworks out there, but decided that I would rather get to know the language first than attempt to learn a framework.

One of the things I got hung up on was how I could redirect to a particular page depending on the logic of the currently executing page (by doing a redirect on the server). An example would be if you access a particular page for which you need to be logged in. At this point, you would want to redirect the user to a logon page. I searched all over and found people asking the same question, even someone asking how to redirect in PHP the way one does in Struts, which I realize now is not a very sensible question, as Struts is a framework, and PHP is a language.