I'd very much recommend Node.js. Very easy to pick up (especially if you already know JS), generally decent documentation all around the ecosystem, package management that doesn't get in your way. Especially when wanting to learn something new (and gain a different perspective to solving problems), JS being half-functional is a very relevant thing. A bit more about that
here and
here.
Mind the inheritance model - Javascript is object-oriented, but not classically-inherited. There are no classes in JS (just a few things that pretend to be classes, with mixed results). Closures are great, and a lot more intuitive (and useful) than their PHP counterpart. Execution model for web stuff is considerably different - long-running process, rather than CGI-style like PHP does. Useful for many non-web things, though not for CPU-bound tasks, things that require extremely low latency (eg. HFT), or things that require arbitrary precision numbers (scientific, financial, ...).
Package management is a big one - nested local dependencies, so no version conflicts. As a result, you have many modular frameworks and libraries, and not so many monolithic ones. This is a good thing, but may be a little unusual when coming from PHP. Generally, you want to be looking for small modules that do one thing only, and just string them together. Definitely use
promises.
Avoid MongoDB, though. It's unfortunately picked up a lot of traction in the Node.js community, but it's utter crap. Similar for Meteor, which relies on MongoDB, and Sails.
Plenty to learn, either way