Sharing session between wildcard subdomain

You are right. I set cookie_domain to some cases:

1. $config['cookie_domain'] = '.localhost:4001' ==> Not working on both.
2. $config['cookie_domain'] = '.localhost' ==> Working on main site only.
3. $config['cookie_domain'] = 'localhost' ==> Main site only.
4. $config['cookie_domain'] = '.127.0.0.1' ==> Main site only.
5. $config['cookie_domain'] = '127.0.0.1' ==> Main site only.
6. $config['cookie_domain'] = 'en.localhost' ==> English site only.
7. $config['cookie_domain'] = '*.localhost' ==> English and French sites.

No idea how to set to make them work on all sites.