Functional programming is a paradigm with origins in lambda calculus and the 1930's. Stack Overflow's developer surveys from both 2017 and 2018 show that the languages optimized for functional programming are low in popularity. However, the same survey recognizes that developers actually working in Elixir, Clojure, F#, and Haskell, actually love their languages much more than PHP or C developers.
So functional programming is unpopular but fun!
The good news is that it can be applied in a popular language like PHP. Whilst the bad news is that PHP might not be really optimized for functional programming, we already have pretty nice libraries of functional primitives for PHP. Accepting functional programming style promises to help PHP developers develop a bit cleaner or more interesting code. More importantly, it will make it a whole lot easier to pick up a purely functional language (Hello, Haskell!) or make use of serverless architectures such as AWS Lambda.