Hi Adithya! Actually, the honest thing is that we all try to follow the hot fashion of the day but that's kind of where people get confused because there are a lot of hot fashions around at the same time as well. If you ask a php expert, he would suggest just go for the php and if you ask someone from ruby language, they would say ruby is fantastic. And same is for python as well. Now Let's have a look at your first question about learning among flask, node and php and thing is php is not framework it is language. Flask is python based and Node is Javascript based runtime environment on server and is not a complete framework. So what I would say in this context is that one should not just jump into a framework directly. Rather playing with a language for couple of months is best thing. Second Question :"which backend programming language should I earn or is important". Answer is No Programming Language is Perfect.Some programming languages are good at one thing but they can be bad at doing something other. Like Speed, Performance, Integration with other tools, Security are the features some Programming languages handle out of the box but each has its trade offs. My advice would be Just pick any of these languages and you should be good with it until unless you stick with good programming style and keep improving yourself. And never switch too much between languages as you learn them, programming needs consistency. Some people try to learn everything at once and that makes people "Jack Of All Trade but Experienced in None". PHP has huge Internet Market Shares, Python is growing and fast, Node is more dynamic but its not framework (Node is JS runtime environment) so all these are good and bad at one or other thing.
Third Advice: " I have some prior knowledge in python and javascript." That's great then. If you already know Python and JS you can learn Any of between Flask and Node but what I have seen is that Learning curve of Node seemed bit steeper to me and it should not be case for you if it seemed to me. But for python I would rather say go with Django rather than flask.
Final Thoughts: Node isn't Framework so if you learn it, you will still have some further framework tools to learn to make a good rapid backend running Like Express or KOA or ADONIS etc. If you go with Flask or Django, they are sort of sufficient to make backend. If you got with PHP, You must go for Laravel because its huge and rapidly growing community now. And yes another options is Rails that is a culture and very mature framework. But kinda slow just my opinion. Moreover you can also look around a bit for what framework can give you more or rapid job if you are looking for a job in your area.