How to properly organize record locking?

Question

PHP

Good afternoon! Suppose we have a table in the database with such a device:

int id
int user_id DEFAULT NULL

And we have a PHP server that takes the ID of the records from the table above and writes to the user_id the ID of the first user that sent the request, for example:

SELECT user_id FROM table WHERE id = ID что нам пришел от клиента
Если user_id == NULL
UPDATE table SET user_id = ID юзера что прислал запрос WHERE id = ID что нам пришел от клиента

Since the server is naturally multi-threaded, there is a possibility that 2 requests from two different clients can come with the same ID almost at the same time. And for both, the user_id == NULL test will be true, and hence 2 UPDATEs will be performed.
Actually interests, how it is possible to avoid a similar unfortunate situation?
There are ideas to use row-level locking on the ID that comes from the client as soon as the request comes in, but it's not clear how to implement it correctly.

Answer 1

Sergey "Why not add the user_id is NULL condition to the UPDATE?"
Here is the perfect answer :)