PHP's problem isn't its age, but that it was intended to do a very specific task in a very specific context, and people who came to it later lack a fundamental understanding of this. If, somehow through magic, PHP was invented today, it would still have this problem, which is the root of most of the other problems with PHP.
I only mention this because some "old stacks" are good, in the sense that they are heavily battle tested, and people still use them because they've been successful over a number of years. You have to be able to use your judgement as a professional to determine whether the move to the "old stack" stems from a good reason or not before deciding to jump.
(I personally would jump if I someone said "let's use PHP instead," but I don't think I would be in a job where PHP could be considered an alternative in the first place. Your situation could be different.)
I only mention this because some "old stacks" are good, in the sense that they are heavily battle tested, and people still use them because they've been successful over a number of years. You have to be able to use your judgement as a professional to determine whether the move to the "old stack" stems from a good reason or not before deciding to jump.
(I personally would jump if I someone said "let's use PHP instead," but I don't think I would be in a job where PHP could be considered an alternative in the first place. Your situation could be different.)