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

Don't bother with figuring out the differences between timezones. That's not necessary.

Whenever the user accesses a page, update a field in their record of the Users table last-updated-time. Then do a query for all users having a last-updated-time within the last 5 minutes. Anything more than this, and they are considered "offline."

If you use your server-time, via the NOW() function in MySQL, you'll side-step calculating differences between timezones.

This is the standard way of tracking how many users are presently online (Meaning, active within the last couple of minutes).

Constantly Updated

If you would like to know they are still active even when they're not jumping from page to page, include a bit of javascript to ping your server every 60 seconds or so to let you know they are still alive. It'll work the same way as my original suggestion, but it will update your records without requiring them to be frantically browsing your site at least once every five minutes.

var stillAlive = setInterval(function () {    /* XHR back to server       Example uses jQuery */    $.get("stillAlive.php");}, 60000);

What you are asking for (after the clarification) is, by definition, impossible. HTTP is a connectionless protocol, so as soon as a user has hit a page and all the content comes back from the server to the user's browser, there is no connection between the two. Someone is "online" with your website for less than a second.

One thing you could do is to have JavaScript on your web page make AJAX requests back to your server on a regular basis which includes identifying information, and a different AJAX request when the user leaves the page, using window.onbeforeunload.

My way may not be the best way but since my site and userbase is all in mysql DB, when a user logins into my site,

1. I update the user table to say theyare online
2. Insert them into an Online table
3. Then I set a session with thecurrent time

Then on every page load I check for the online time session, if it exist, I check to see how old it is, if it is less then 5 minutes old, I do nothing, if it is older then 5 minutes, then I update the session time again with current time and also update the online users table with the time

Then I have a cron job that runs every 10 - 15 minutes that deletes any uses from online table and marks there user table as offline if there online time has updated within X amount of minutes