//Example Login Code:
if($username=="ok" && $password =="ok"){
....//Create login sessions and send cookies

if(!isset($_SESSION['time'])){//Save login datetime to the session
$_SESSION['time']=time();
}

}

//Example Logout Code:
$duration=time() - $_SESSION['time'];//calculate duration
echo "Time user was logged in is".$duration;
session_destroy();