public function start(string $key)
    {
        $filteredKey = $this->filterKey($key);
        $value = $this->get($filteredKey);
        if ($value === null) {
            $this->lastKey = $filteredKey;
            ob_start();
        } else {
            $this->lastKey = null;
            trigger_error('Cache start failed');
        }
        return $this;
    }