How to track Visit Duration for 1 page website

Question

I have a website page where most of the visitor will spend their time on. The site uses PHP with Codeigniter and jQuery.

Problem: However, I believe tracking systems like Piwik calculate the time spent on a page by subtracting the time the visitor leaves that page to a new page from the time he loads the page.

Is there a way around this? Maybe use javascript to trigger the tracking system so it knows that the visitor is leaving the page?

Answer 1

Edit: I added some "salting" to the $id, I don't know.. I get the feeling that it's even more unique, even though I think it's a bit unlikely that there will be similar IDs due to the use of micro seconds but just to be extra sure

Something like this
JQuery: Ajax request to StayAlive.php every 10 seconds

function pingPing(){
    $.ajax(
    {
        type:'GET',
        url:'StayAlive.php', 
        success: function() 
        {
            setTimeout(pingPing, 10000);
        }
    }
}

PHP: StayAlive.php

<?php
function newUser(){
  $id=sha1($_SERVER['HTTP_USER_AGENT'].microtime().$_SERVER['REMOTE_ADDR']);
  setcookie("id", $id, time()+3600*24*365);
  //create new row in the database for that $id

}
if (isset($_COOKIE["id"])){
  //check if the id is in the database
  //if yes => time_spent+=10
  //if no => newUser();
}
else
  newUser();
?>