src/Payment/API.php
protected function getSignkey($api)
    {
        return $this->sandboxEnabled && $api !== self::API_SANDBOX_SIGN_KEY ? $this->getSandboxSignKey() : $this->merchant->key;
    }
protected function getSignkey($api)
    {
        return $this->sandboxEnabled && $api !== self::API_SANDBOX_SIGN_KEY ? $this->getSandboxSignKey() : $this->merchant->key;
    }
Return key to sign.

@param string $api

@return string