I don't think PHP is going anywhere, lots of new projects are still being started with PHP but less of them are project that you'll hear about, 7 made huge overdue changes. I think python is a good choice although personally I'm not a huge fan of the language, lots of people I know love it, even I love how fast I can put stuff together. Learning node is good if you plan to go full stack as then you only need javascript / typescript for both front and back which is an advantage but I don't see it ever becoming the mainstream go to language. My latest projects have been in node and I quite like it, but I still dislike javascript. Another option if you dont hate microsoft is C#, I'm not a fan of microsoft but I do like C# for really big projects.
Other options are Java (I quite like Java for big APIs, that was a few years back now though), Ruby (not used it much so can't comment, everyone went mad for it about 10 years ago, then went quiet, seems to be getting louder again), Go (very fast to learn once you know other languages, seems quite good, still waiting to use it on a real project), Rust (a serious language for serious projects, looking forward to using this one for real stuff)
Apologies if I've missed your favourite backend web language, these are the ones I'm familiar with at least to a basic level.