I'd say try to just find another job, period. Preferably one where you're using the new stack you want to learn, or at least where they're open to letting you learn it and try it out. Or maybe it'll be with PHP still, but it'll be a good place to work - or at least one you can tolerate while you're learning other stuff.
The main reason I suggest that over taking time off to learn is that, in my experience, trying to learn an entirely new stack is just about useless without having a real project to back it up. I've started so many tutorials on so many languages and platforms that I'm probably one of the best "hello world" coders around, but I can't do anything useful in anything. I need an actual project, with team members who know the tech, and with actual goals I'm trying to accomplish. Cool, I can write a ToDo app on the MEAN stack; I can't do anything else with Angular because I have no knowledge of it's capabilities and I have no ongoing need to.
That and it's always best to keep in the swing of things. Being a developer isn't just coding, it's also working with others (even in a crappy environment), and if you take time to just focus on code you lose out on that.