ini_set('session.gc_maxlifetime', 18000);

var refreshSn = function ()
{
    var time = 600000; // 10 mins
    settimeout(
        function ()
        {
        $.ajax({
           url: 'refresh_session.php',
           cache: false,
           complete: function () {refreshSn();}
        });
    },
    time
);
};

<?php
session_start();

// store session data
if (isset($_SESSION['id']))
$_SESSION['id'] = $_SESSION['id']; // or if you have any algo.
?>