How to implement similar functionality?

Question

PHP

There is some abstract application (web, mobile application).
After authentication, the user starts a timer. Only one timer can be active. The time range when the timer starts is known. If the user switches to another application (for example, mobile), he should get into his session.
So far, from ideas: when starting the timer, put a timestamp (startTimer) and a sign (taskComplete) in the database, and check it at startup.
PHP backend using RESTful API.

Answer 1

cookies?

Answer 2

Does it work