cross_subdomain_session.php

<?php 
// Its recommended to place this in a file that is then included in all subsequent files, 
// as without this information the page will not be able to access the proper session.

session_name("2620368ghwahw90w"); // Set to whatever arbitrary value you would like. The more complex the more secure
session_set_cookie_params(0, '/', '.mydomain.com'); // Set to the base level of the domain you wish to use
session_start(); // Start the session