code: public static function allowFrom(string $allowFromUrl):self
      {
          $middleware = new self(sprintf(self::VALUE_ALLOW_FROM, $allowFromUrl));
  
          if (!filter_var($allowFromUrl, FILTER_VALIDATE_URL)) {
              throw new MiddlewareException(
                  $middleware,
                  sprintf("'%s' is not a valid URL", $allowFromUrl)
              );
          }
  
          return $middleware;
      }
docs: Allow frame from a given URL.
  
  @param string $allowFromUrl
  @return self
  @throws MiddlewareException