public function getRedirectUrl($option, $payKey)
    {
        $url = $this->config['gateway_url'].'?cmd=';

        if ($option == 'approved') {
            $url .= '_ap-payment&paykey='.$payKey;
        } elseif ($option == 'pre-approved') {
            $url .= '_ap-preapproval&preapprovalkey='.$payKey;
        }

        return $url;
    }