Whenever you Google around for things like "Who's Online php", you'll find that a lot of the solutions are centered around using a database. However, is this really necessary? [...] One goal here: get Who's Online functionality off of the database. We'll explore a possible solution with Memcached that I've personally implemented, and thus far, it's been working great.

His alternative, using the memcache functionality that can be enabled in PHP, stores a "user ID" (either a real user ID or just some unique identifier) and their last activity time. This information is stored in a local file that can be queued each time the page it loaded. The other piece is a backend script that goes through this same file and clears out the old values. Depending on the size of the file/traffic of the site, this process could even be integrated into the same script that writes it.