I commented separately, I use sockets for this. I don't know about libraries for PHP as I haven't used it in a while, but most socket libraries make it easy to implement something.

My implementation is in-memory, and the nice thing about it is that if fit example my Node.js app restarts, all active users will get disconnected, and they will emit a connection event, allowing me to get the count of online users again.

I can extract relevant code and share it if it'd help.