code: protected function ifNoneMatch(Request $request, Response $response, string $etag): Response
      {
          $ifNoneMatch = $request->getHeaderLine('If-None-Match');
          if ($ifNoneMatch && $etag === $ifNoneMatch) {
              return $response->withStatus(304, "Not Modified");
          }
          return $response;
      }
docs: Checks the `If-None-Match` 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 string $etag  The ETag for comparison
  @return \Psr\Http\Message\ResponseInterface  The same or new response