src/MessageHelper.php
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;
    }
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;
    }
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