public function getCacheTimestamp($key)
    {
        if ($this->sqlInitialized()) {
            $_key = $this->sql->real_escape_string($key);
            if ($response = $this->sql->query("
                SELECT *
                    FROM `{$this->table}`
                    WHERE
                        `{$this->key}` = '$_key'
            ")) {
                if ($response->num_rows > 0) {
                    return new \DateTime($response->fetch_assoc()['timestamp']);
                }
            }
        }
        return false;
    }