PHP / MySQL - how to prevent two requests *Update

Question

I have some question ... example: a user will buy something for his USD

1. Check his USD Balance
2. Deduct the USD from his account
3. Make an Order -

Answer 1

First off, you have to use transactions, but that's not enough. In your transaction, you can use SELECT FOR UPDATE.

It's basically saying, "I'm going to update the records I'm selecting", so it's setting the same locks that an UPDATE would set. But remember this has to happen inside a transaction with autocommit turned off.