If a PHP session ($_SESSION) is not being passed from a web site using WordPress.
ANSWER: Add the missing “www.” in the wordpress site area.
WRONG URL: http://example.com (missing the “www.”)
RIGHT URL: http://www.example.com
By adding the “www.” to the WordPress domain name the problem was solved:
Login to WordPress Admin area
Go to Settings – General
Add “www.” to the WordPress address (URL) and Blog address (URL).