If your IP changes, it doesn’t matter as you are using the HTTP_HOST setting which uses the domain name. HTTP_HOST is a standard $_SERVER item and will be populated by any standard Apache installation. The only time I have known it to be missing is accessing PHP via the command line and of course I had no expectation for it to be there.

I thank you for your suggestions but you have clearly misunderstood what this tutorial was meant to achieve. Perhaps you could write up a tutorial explaining your goals and methods for us to publish here?

Update: Thinking about it the example already shows you how different domains can access the same content or different content. It is all based on the cases set in the database.php config file. example.com and http://www.example.com could be replaced with examplea.com and exampleb.com. They would be different domains looking at the same data, while the other cases in the switch are looking for different domains and different data.

And for the $_SERVER['HTTP_HOST'] solution:

This solution won’t work for multidomain hosts and for raw IPs and even if you fill in all possible values you can’t be sure IP is still valid after some time (it can change).

The most simple solution is to add site designation into small text file (ie. site/site.txt) or even a folder name. Every time you update site files this one doesn’t change. Even when the site is accessed using other valid domain name the site works because it doesn’t guess. It still works even if IP changes.