public function getRequestHeader($header)
    {
        $header = str_replace('-', '_', strtoupper($header));
        $value = $this->{'HTTP_' . $header};

        if (!$value) {
            $headers = $this->getRequestHeaders();
            $value = !empty($headers[$header]) ? $headers[$header] : null;
        }

        return $value;
    }