I think a point I would like to put forward is that I don't believe php is going anywhere — and more importantly — being replaced. Yes, it's older, but it's well established and rooted even in today's "framework-crazy" Dev environment.
As far as what to learn next? There are no rules. I believe that anything learned is a benefit. You may uncover concepts from learning an older language that might be lost trying to learn the newest and fanciest releases.