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;
    }
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
Return key to sign .