Have you requested this out_trade_no?

This out_trade_no is the order number on your own website. The process is like this

The user pays on your website. Your own website first generates an order number, such as 1234, and then requests the relevant information to WeChat. This out_trade_no is also your order number (1234). Then WeChat will call back after the payment is successful, and it will keep this out_trade_no. Here you go, you can use this to identify who has paid successfully

Your picture

See, that is an example, you still annotated it, so that $out_trade_no It is empty.

The order number for merchant payment is generated by the merchant. WeChat Payment requires the merchant's order number to be unique (it is recommended to generate the order number based on the current system time plus a random sequence). To re-initiate a payment, use the original order number to avoid repeated payments; payment cannot be re-initiated for an order number that has already been paid or has been called for order closing or cancellation (please see the API list below).