PHP : Auto expire session / Auto logout after specific time

gistfile1.php

<?php
//on pageload
session_start();

$idletime=60;//after 60 seconds the user gets logged out

if (time()-$_SESSION['email']>$idletime){
    session_destroy();
    session_unset();
    header('Location: ../index.php');// relocating user to home page

}else{
    $_SESSION['email']=time();
}

//on session creation
$_SESSION['email']=time();