As index.php and the front-end architecture diagram show, Cache.php is loaded before the application. If a cache check succeeds, the system exits without loading the application.

if($cache->isCached()){
                      $cache->response($cache->buffer);
                    exit;
                  }

If the resource is not cached, the page must be processed to produce the response. The resulting response is then cached to avoid processing it again on the next request.

$cache->set($buffer);

If the resource is not cached, the page must be processed to produce the response, and the newly requested resource is then saved.