Assume that you are doing a banking application. If users are logged into your site, how to detect their inactivity and ask them to log out if they remain inactive for a period of time? Inactive here means they have either switch to other tabs, or not touching the browser application.
I guess think I can do this by registering every mouse movement or keyboard movement when users are doing on EVERY page of my application. But the code would be very ugly and hard to maintain. Is there other more elegant ways of doing this?
If the user is requesting new pages/data from your server on a regular basis, then adjusting the session timeout in PHP should work for this (assuming you are using PHP sessions).
If the concern is that they could be sitting on one page for a good length of time with no trips to the server (e.g. filling out a long form), and you want to distinguish between this and the user simply switching to another window, you could do something like use javascript to request some data using XMLHTTPRequest every five minutes or so to keep the session alive. You could use the window.focus and window.onblur events in javascript to stop and restart this mechanism (I think there are some differences for IE, there is a good explanation here).