Whats the easiest way to determine if a user is online? (PHP/MYSQL)

Question

Is there a way I can piggy back sessions to know if the user is online?

I.e: use logs on, I set a $_SESSION variable, user times out- cookie Garbage collector update

Answer 1

You can also use the onunload tag... On the body More information here (https://www.w3schools.com/jsref/event_onunload.asp)

This way you don't have to wait 4 -5 minutes for the user to get offline.

But you should still use the ajax update method in-case the user's browser crashes...

Edit: As @Quentin pointed out in the comments you may not be able to send out requests but there is a way you can do it... Explained in more detail in this Article.
So basically they are using the sendBeacon() Here is an Example

window.addEventListener("unload", function logData() {
  navigator.sendBeacon("/log", analyticsData);
});

NOTE: Although it should be kept in mind that this should not be the only method you should depend upon, and should implement for redundancy...