This article series (continued in Implementing MVC in PHP: The Controller, Implementing MVC in PHP: The View, and Implementing MVC in PHP: The Model) demonstrates how to build an MVC web framework using PHP 5. This article covers the basics of MVC web frameworks, building the foundation classes for a framework that the other three articles in this series will build.

With the introduction of PHP 5 and its new OOP features developers can now seriously talk about building solid APIs and more complex MVC frameworks for the web in PHP. This was possible before, but the new features in PHP 5 make it easier to integrate more advanced features into MVC frameworks, such as SOAP and WSDL.

In this article I assume that you have a solid understanding of object-oriented programming and that you have at least scanned the upcoming changes to the OOP structure of PHP in PHP5.
What is MVC?

MVC is the idea that you have three different pieces that work in unison to form a complex application. A car is a good real-world example of MVC. With a car you have two views: the interior and the exterior. Both take input from the controller: the driver. The brakes, steering wheel and other controls represent the model: they take input from the controller (driver) and hand them off to the views (interior/exterior) for presentation.
MVC on the Web

The ideas behind MVC frameworks are quite simple and extremely flexible. The idea is that you have a single controller (such as index.php) that controls the launch of applications within the framework based on arguments in the request. This usually includes, at a minimum, an argument defining which model to invoke, an event, and the usual GET arguments. From there the controller validates the request (authentication, valid model, request sanitization, etc.) and runs the requested event.

http://www.onlamp.com/pub/a/php/2005/09/15/mvc_intro.html