Devarticles.com has always been good about producing quality articles, especially when it comes to PHP - and one of their latest articles follows that tradition.

Developing Multipurpose Pages With PHP dives headfirst into walking you through multipuprose pages that, based on whatever condition they're called with, can display just about anything that you want them to. Most sites (including this one) are based off of templates anyway, so it's not a hard transition to these multipurpose pages.

In their examples they use the situation of having a forum that can be used by both members and non-members, with a simple change to the select statements.