Symfony\Component\HttpKernel\HttpCache\HttpCache::validate PHP Method

    protected function validate(Request $request, Response $entry, $catch = false)
    {
        $subRequest = clone $request;
        // send no head requests because we want content
        if ('HEAD' === $request->getMethod()) {
            $subRequest->setMethod('GET');
        }
        // add our cached last-modified validator
        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified'));
        // Add our cached etag validator to the environment.
        // We keep the etags from the client to handle the case when the client
        // has a different private valid entry which is not cached here.
        $cachedEtags = $entry->getEtag() ? array($entry->getEtag()) : array();
        $requestEtags = $request->getETags();
        if ($etags = array_unique(array_merge($cachedEtags, $requestEtags))) {
            $subRequest->headers->set('if_none_match', implode(', ', $etags));
        }
        $response = $this->forward($subRequest, $catch, $entry);
        if (304 == $response->getStatusCode()) {
            $this->record($request, 'valid');
            // return the response and not the cache entry if the response is valid but not cached
            $etag = $response->getEtag();
            if ($etag && in_array($etag, $requestEtags) && !in_array($etag, $cachedEtags)) {
                return $response;
            }
            $entry = clone $entry;
            $entry->headers->remove('Date');
            foreach (array('Date', 'Expires', 'Cache-Control', 'ETag', 'Last-Modified') as $name) {
                if ($response->headers->has($name)) {
                    $entry->headers->set($name, $response->headers->get($name));
                }
            }
            $response = $entry;
        } else {
            $this->record($request, 'invalid');
        }
        if ($response->isCacheable()) {
            $this->store($request, $response);
        }
        return $response;
    }