By default, PHP uses the 'PHPSESSID' cookie to propagate session data across multiple pages, and by default it uses the current top-level domain and subdomain in the cookie declaration.Example: www.domain.comThe downside to this is that the session data can't travel with you to other subdomains. So if you started a session on www.domain.com, the session data would become unavailable on forums.domain.com. The solution is to change the domain PHP uses when it sets the 'PHPSESSID' cookie.Assuming you have an init file that you include at the top of every PHP page, you can use the ini_set() functi...