src/MessageHelper.php
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;
    }
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;
    }
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