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.

One of the implications of following the rule is related to the whole www vs no-www question. Basically you should always use www if you're planning to use any other sub-domains and you want them cookie-free. This is because you have no way to set a cookie only to the top-level domain.

His code sets cookies for the ".phpied.com", "phpied.com" and "www.phpied.com" domains. Since the domains without the "www" offer a bit broader range of possible matches, he recommends that if you're ever going to use any kind of subdomain, you'll want to use the more broad cookie to cover it.