JQuery :: Throw Script When User Leave Website

I want to change a db record when the user leave the website. I have this PHP script:
<?php
require_once("../config.inc.php");
if (isset($_POST["user"]) && isset($_POST["ip_address"])){
$query = 'UPDATE users SET logged = 0 WHEREuser= "' . $_POST["user"] . '" AND ip_address = "' . $_POST["ip_address"] . '" AND confirmed = 1;';
mysql_query($query, DB_LINK);
}
?>

And this jQuery script:
// script da includere in tutti i file delle pagine per l'utente loggato
// cambia lo stato dell'utente da loggato a non-loggato quando viene chiusa la finestra del browser
$(window).unload(function(){
$.ajax(AJAX_PATH + "logout.php", {type: "POST", data: "user=" + escape(USER) + "&ip_address=" + escape(IP_ADDRESS), success: function(){alert("Arrivederci!"); return true;}});
});
but it doesn't seem to work.