Omnipay\Alipay\Requests\LegacyExpressPurchaseRequest::validateParams PHP Method

    protected function validateParams()
    {
        $this->validate('_input_charset', 'out_trade_no', 'subject', 'payment_type', 'total_fee');
        $this->validateOne('seller_id', 'seller_email', 'seller_account_name');
    }