Sharing Cookies between Sub-domains

If you have two sites using the same domain and would like to share cookies between them, set something like this in your settings.php file for each domain:

ini_set('session.cookie_domain', '');

Be sure you include the leading '.' before the domain name or it won't work.

This allows users to maintain login status between any sites configured for domain-wide cookies.

This can also have negative side effects, so don't do this unless you're familiar with all the cookies involved for the sites you want to share cookies between.