How to properly organize balance write-off?

Question

PHP

Let's take this service as an example:
A user registers on a site where he can replenish his account and create a community, a guest of this community can request a certain service that the community creator pays for.
Let's implement it like this:

$this->balance -= $sum;
$this->save();

+ there is a record in another table (reason, amount, balance)
Is this correct? Let's say there are 100500 guests, N users request the service at the same time, will the funds be debited correctly? If not, how to do it right?

Answer 1

If your operation is NOT ATOMIC, then you need to wrap it in a transaction
Partially described here - php.net/manual/en/pdo.transactions.php