The cookie used to store the session id sets, by default, the cookie domain to the current domain. This means that if you are visiting www.domain.com, the session id will be stored in a cookie bounded to the domain www.domain.com. This behavior leads to a problem: if your site uses multiple subdomains, the session won’t be shared between the site’s subdomains.

To solve this problem you have to share the cookie between all subdomains, setting its domain it to “.domain.com”. The following code shows how to change the session cookie domain with Zend Framework:

Zend_Session::start(array('cookie_domain' => '.domain.com'));

Browers compatibility

The prefix dot in “.domain.com” is not always necessary, but it’s highly recommended to support all browsers.

References

• PHP session cookie multiple domains