private function signature()
    {
        if (isset($this->apiKey)) {
            $apiKey = $this->apiKey;
        } else {
            $apiKey = \RpayLite\RpayLite::$apiKey;
        }
        $array_payload = array();
        array_push($array_payload, $this['key']);
        foreach ($this['items'] as $item) {
            array_push($array_payload, $item['id']);
            array_push($array_payload, $item['name']);
            array_push($array_payload, $item['unit_price']);
            array_push($array_payload, $item['quantity']);
        }
        if (isset($this['cart_id'])) {
            array_push($array_payload, $this['cart_id']);
        } else {
            array_push($array_payload, "");
        }
        $payload = join(";", $array_payload) . ";";
        return hash_hmac("SHA1", $payload, $apiKey);
    }