mariadb is a fork of MySQL so the same issue exists there https://database.guide/what-is-the-year-2038-problem/
From the coding PHP coding side it really is a trivial fix, BUT have you found every place where that code might be.
Where someone hacked something quickly together and left it because it worked.
It's not the glaring issues that are the problem, it the stuff that is embedded or used everywhere that is the problem.
If you are in IT, what are you doing tomorrow: https://www.zdnet.com/article/openssl-warns-of-critical-security-vulnerability-with-upcoming-patch/
Probably dealing with that.....
-Dave