Set wordpress cookie as root cookie for your domain and all subdomains

gistfile1.txt

// If you include this in your functions.php, the wordpress cookie is set for the current domain
// and all subdomains

class RootCookie { 
    private static $cookie_name;

    static public function init() {
        self::$cookie_name = "wordpress_logged_in_" . md5(get_site_url());

        if ( is_user_logged_in() ) {
            self::setRootCookie();
        }
        else {
            self::removeRootCookie();
        }
    }
    
    static function getDomainWithoutSubdomain() {
        $domain = parse_url(get_site_url(), PHP_URL_HOST);
        $domain = explode('.', $domain);
        $domain = array_reverse($domain);
        $domain = "$domain[1].$domain[0]";

        return $domain;
    }    
    
    static function setRootCookie() {
        $domain = self::getDomainWithoutSubdomain();
        setcookie(self::$cookie_name, $_COOKIE[self::$cookie_name], 0, "/", $domain, false, true);
    }

    static function removeRootCookie() {
        $domain = self::getDomainWithoutSubdomain();
        setcookie(self::$cookie_name, "", time()-1000, "/", $domain, false, true);
    }    
} 

RootCookie::init();