PHP and Node.js happen to be the two languages/platforms I am most proficient with. I don't think one can be completely replaced by the other for all situations

PHP and Node.js happen to be the two languages/platforms I am most proficient with. I don't think one can be completely replaced by the other for all situations. It really depends on what you are planning to do with it. If you use PHP to develop websites/apps and you aren't facing issues with scaling/load, you can continue with PHP, which has a bigger ecosystem compared to Node.js. If you want to deploy a real time app or a socket or tcp server, or even a high load API, you can give Node.js a shot. Node.js can do normal web stuff too, but I don't know any good reason why you would want to learn and use it when you are already proficient with PHP. For the sake of simply learning it, just go ahead and learn it.