public static function getHeader($header, $default = null)
    {
        $header  = strtolower($header);
        $headers = self::getInstance()->getHeaders();
        $headers = array_change_key_case($headers, CASE_LOWER);
        if (array_key_exists($header, $headers)) {
            $value = \is_array($headers[$header]) ? implode(', ', $headers[$header]) : $headers[$header];
            return $value;
        }
        return $default;
    }