Better to learn PHP first or go straight to Rails?

I have a very rudimentary knowledge of programming, I know front-end development reasonably well, and now looking to pick up a back end language to build a web-application.

I've heard from people, that to build an actual understanding of how these things work, it is better to learn a language like PHP before going on to frameworks like Ruby on Rails. Now I'm stuck deciding whether to learn PHP or Ruby on Rails.

Also, which would be more beneficial for a future job in web-application development?? I've heard Rails devs will earn more, however this could just be speculation.