public function getPayUrl(array $bill, $successUrl)
    {
        // Preset required fields.
        $bill = array_replace(
            ['payUrl' => null],
            $bill
        );

        $payUrl = parse_url((string) $bill['payUrl']);
        if (true === array_key_exists('query', $payUrl)) {
            parse_str($payUrl['query'], $query);
            $query['successUrl'] = $successUrl;
        } else {
            $query = ['successUrl' => $successUrl];
        }

        $payUrl['query'] = http_build_query($query, '', '&', PHP_QUERY_RFC3986);

        return $this->buildUrl($payUrl);

    }