protected function setTtl($expiration, $contents, $cacheFile)
    {
        switch ($this->ttlStrategy) {
            case 'embed':
                $contents = ($expiration ?? PHP_INT_MAX) . "\n" . $contents;
                break;
            case 'file':
                if (isset($expiration)) {
                    file_put_contents("$cacheFile.ttl", $expiration);
                }
                break;
            case 'mtime':
                // nothing
                break;
        }

        return $contents;
    }