code: protected function ifModSince(Request $request, Response $response, int $timestamp): Response
      {
          $ifModSince = $request->getHeaderLine('If-Modified-Since');
          if ($ifModSince && $timestamp <= strtotime($ifModSince)) {
              return $response->withStatus(304, "Not Modified");
          }
          return $response;
      }
docs: Checks the `If-Modified-Since` header, maybe sending 304 Not Modified.
  
  @param \Psr\Http\Message\ServerRequestInterface $request  The HTTP request
  @param \Psr\Http\Message\ResponseInterface $response  The HTTP response
  @param int $timestamp  The timestamp for comparison
  @return \Psr\Http\Message\ResponseInterface  The same or new response