This is really handy for those of us who have the same code handling multiple sites or multiple sub-domains. A case in point: When I coded NetBoardz (my free forum hosting service now defunct), I had one codebase handling all 250 forums. How? Simple. When the code runs, it determines which site the user is loading and does different things (like using different databases) dynamically.

He shows the two key points to dividing things up - grabbing the domain and subdomain they were trying to access. This can be fed into a script that can switch things like layout or even functionality based on which site they've chosen.