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

This is how I used to do it many years ago..

results = query("UPDATE table SET value=value-5 WHERE value>=5 AND ID=1")
if (results == 1) YEY!

(Is this still a reliable method?)