Unable to access session variable in php script called using ajax

[FedEx]

Consider the following scenario. I have three php files, file1.php, file2.php and file3.php located on my server.file1.php starts a session and sets a session variable say,var.I am able to access var using $_SESSION['var'] in file2.php. file3.php is called using jquery ajax functionality, but im unable to access $_SESSION['var'] in file3.php. if i do a gettype($_SESSION['var']) in file3.php it returns NULL.What could be the problem here ?Please helpThank You