On the TutToaster site today there's a new tutorial showing you how to create an entire PHP forum completely from scratch using PHP and a MySQL backend.

The code I’ll show is exactly the same than the one you can download, except for some comments. In the original project you’ll have everything well documented (using phpDocumentor). [...] This project doesn’t follow a MVC pattern, but we’ll use classes and try to separate the different functionalities.

They walk you through the creation of the database (a "posts" table) and how to organize the file structure, configuration, making the header and footer as well as the classes to do things like database connections, validation and creating the output (view) as well as working with the posts itself.