public static function getAuthorizationHeader()
    {
        $headers = null;
        if (isset($_SERVER['Authorization'])) {
            $headers = trim($_SERVER["Authorization"]);
        } else if (isset($_SERVER['HTTP_AUTHORIZATION'])) {
            $headers = trim($_SERVER["HTTP_AUTHORIZATION"]);
        } elseif (function_exists('apache_request_headers')) {
            $requestHeaders = apache_request_headers();
            $requestHeaders = array_combine(array_map(
                'ucwords',
                array_keys($requestHeaders)),
                array_values($requestHeaders)
            );
            if (isset($requestHeaders['Authorization'])) {
                $headers = trim($requestHeaders['Authorization']);
            }
        }
        return $headers;
    }