public function invalidate()
    {
        $script = escapeshellcmd($this->rootDir . "/../{$this->cacheScript}");
        $urlsFile = escapeshellarg($this->createUrlsTempFile());
        $curlTimeout = escapeshellarg($this->curlTimeout);
        // Execute in background.
        $process = new Process(sprintf('%s %s %s', $script, $urlsFile, $curlTimeout));
        $process->start();

        $this->resetCache();

        return $urlsFile;
    }