Question

Is there a way to use javascript to determine how long someone was looking at my webpage before they closed their browser or hit the back button? Something like send a message to php page every few seconds or so in the background?

Answer 1

Start a timer when the page is loaded and when the page is unloaded, stop it.

var timeSpent = 0; //seconds on page
var timer;
window.onload = function() {
  timer = setInterval( function() { timeSpent++; }, 998 );
};

window.onunload = function() {
  timer = clearInterval( timer );
  //.. do something with timeSpent here...
}