I think every programmer should learn different languages and work in different platforms. You can ask me why? If you work on PHP backend development for 5 years, you will become an experienced guy and will earn better income.
So why you’ll invest your time for other language and other platform?
Because it will give you a good taste of programming and you’ll enjoy yourself. I’m not telling you to leave PHP development and move to other platform, I’m just telling you to make some extra time from