From the Limitless Studios website, there's a new tutorial for those frustrated by the other "who's online" tutorials that require a database. Their example doesn't, opting instead to handle the process with PHP's sessions.

Most web developers will usually like to determine the number of online users currently browsing their web site. Most tutorials will teach you how to do just that using a database of some sort because it is easy to implant. Using a database though can be a bit slow at times, especially if your web site gets a lot of hits. In this tutorial, we will teach you an alternative and effective way to determine the number of online users WITHOUT using a database.

They note the overhead that using a database can cause, and look instead to something PHP works with natively - sessions. Their code is contained inside of a function, checking the session path PHP uses for session files that haven't expired yet.