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

Use TRANSACTION and if it fails you can rollback.

For example, assume the current balance is $20.

Connection A               Connection B
=======================    ===========================
BEGIN TRANSACTION         
                           BEGIN TRANSACTION
SELECT AccountBalance  
                           SELECT AccountBalance
--returns $20
--sufficient balance,
--proceed with purchase
                           --returns $20
                           --sufficient balance,
                           --proceed with purchase

                            --update acquires exclusive lock
                           UPDATE SET AccountBalance
                              = AccountBalance - 20
--update blocked due
UPDATE SET AccountBalance
  = AccountBalance - 20

                           --order complete
                           COMMIT TRANSACTION

--update proceeds

--database triggers
--constraint violation
--"AccountBalance >= 0"

ROLLBACK TRANSACTION