Sharing session between wildcard subdomain

Found a way to handle all sites:

PHP Code:

$subdomain = array_shift((explode(".",$_SERVER['HTTP_HOST'])));
if (in_array($subdomain, array('en','fr'))) {
    $config['cookie_domain'] = $subdomain . '.localhost';
} else {
    $config['cookie_domain'] = '.localhost';
} 


But it is like each subdomain and main site have their own different flow of session, but not share the same and only one session for all.