static public function GetRootDomain($domain=null) 
	{
	    $domain = !empty($domain) ? $domain : (isset($_SERVER['HTTP_HOST']) ? $_SERVER['HTTP_HOST'] : null);
	    if (!$domain) {
	        return 'localhost';
	    }
	    
	    $re_domain = '';
	    $domain_postfix_cn_array = array("com", "net", "org", "gov", "edu", "com.cn", "net.cn", "cn");
	    $array_domain = explode(".", $domain);
	    $array_num = count($array_domain) - 1;
	    if ($array_domain[$array_num] == 'cn') {
	        if (in_array($array_domain[$array_num - 1], $domain_postfix_cn_array)) {
	            $re_domain = $array_domain[$array_num - 2] . "." . $array_domain[$array_num - 1] . "." . $array_domain[$array_num];
	        } else {
	            $re_domain = $array_domain[$array_num - 1] . "." . $array_domain[$array_num];
	        }
	    } else {
	        $re_domain = $array_domain[$array_num - 1] . "." . $array_domain[$array_num];
	    }
	    return $re_domain;
	}