Load a PHP session only when needed http://blog.lavoie.sl/2012/12/properly-create-and-destroy-php-session.html

global.php

<?php
# global.php
# In some global configuration file,
# Start the session only if the cookie is present
if (isset($_COOKIE[session_name()])) { 
  session_start(); 
}
# In the same file or elsewhere, after the previous lines:
if (isset($_SESSION['userid'])) { 
  # If the session has been loaded and the userid is detected, set it
  $userid = $_SESSION['userid']; 
} 

login.php

<?php 
# login.php
# We start a session if it not already started
session_id() == '' && session_start(); 
$_SESSION['userid'] = '1'; 

logout.php

<?php
# logout.php
session_start(); 
# Destroy the session, this is for the current request
session_destroy();
# Ensure that the cookie is destructed by setting a date in the past
setcookie(session_name(), false, time() - 3600);