I have looked at a few topics (here & google) regarding detecting browser exit in php and im not really any clearer on how to do so.
I tried the register_s
Not just with PHP.
PHP runs server-side, and is far done processing your page by the time the user will have a chance to close their browser. You could technically detect if PHP was still processing the page and the user closes it, with a specific configuration. However, it is not ideal. See connection_aborted().
What you need to do is set up a long-polling connection with JavaScript, and monitor it server-side. You will then get an idea for when that window is closed. That connection could be made to your PHP script, allowing PHP to check connection_aborted(). Note that you will need to set up ignore_user_abort() for this to work, or configure PHP.ini accordingly.