If you already know PHP then it will be easier to pick up Ruby first, and when you know Ruby it may be easier to get into Elixir as Elixir’s syntax was in part inspired by Ruby 
For FP, reading some of the Elixir books will help, like @ulissesalmeida’s Learn Functional Programming with Elixir Book Portal | Devtalk 
Here’s my review of it on EF: