Adds $_SESSION-like functionality to WordPress. Every visitor, logged in or not, will be issued an instance of WP_Session. Their instance will be identified by an ID stored in the _wp_session cookie. Typically, session data will be stored in a WordPress transient, but if your installation has a caching system in-place (i.e. memcached), the session data might be stored in memory. This provides plugin and theme authors the ability to use WordPress-managed session variables without having to use the standard PHP $_SESSION superglobal.