Can I ask all the questions at once =,=

Can you read the document carefully? Refund needs to pass an order number parameter. You can pass the order number and it will be ok. Pass the order number and merchant number to Alipay and it will match the total amount of money that needs to be refunded. Pay attention to your own transaction fees. After the transaction limit is exceeded, a fee will be charged for each transaction.

$params = array();
$params['detail_data'] = '支付宝交易订单号^退款金额^退款描述#第二笔支付宝交易订单号^退款金额^退款描述';

// 退款总笔数
$params['batch_num'] = '2';

After successful payment, save your Alipay serial number - -#

Wouldn’t it be more reasonable to ask the official Alipay about this kind of question...

The money will be refunded to whoever paid, back to the original route, that is, if the user paid with a bank card, the money will be refunded to the bank card.

return_url or notify_url will send you the user number