Log Visitor Visit Length

this is a simple script, that uses ajax that sends to the server the total amount of time (in seconds) that the visitor was reading or whatever.

var startime=(new Date()).getTime();window.onunload=function(){
var x=(window.ActiveXObject)?new ActiveXObject('Microsoft.XMLHTTP'):new XMLHttpRequest();
x.open("GET","count.php?t="+(((new Date()).getTime()-startime)/1000),true);x.send(null)}

that's the javascript...

<?php $f=fopen("visitLength.txt", 'a');fwrite($f, $_REQUEST["t"].";
");fclose($f);?>