PHP session is getting reset between subdomains
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I have a website running with two subdomains, both of which require login (based on the same DB access credentials). In order to make it easier for users, I wanted to change it so they can navigate both subdomains without having to log in separately: essentially, they log in at one of the subdomains and can then freely navigate between one and the other.

One solution I found at http://stackoverflow.com/questions/644920/allow-php-sessions-to-carry-over-to-subdomains involves changing the session.cookie_domain variable to so that all subdomains would share the session variables, but something seems to be wrong. I can still login at subdomain1 and navigate it, but as soon as I load a page from subdomain2, subdomain1 instantly loses all its session data and I'm taken back to the login page. This also happens the other way around (logging in from subdomain2 at first). Prior to the change, subdomains could be simultaneously logged in but they wouldn't 'see' each other.

What could be causing this problem to occur?