I agree with what @farebord says!
Even though there are a lot of alternatives. You should definitely learn a framework. For PHP one of the big ones is Laravel.
If you want to try something I can recommend Ruby. And for Ruby you should try out Ruby on Rails.
But since you have experience of front-end development you probably feel home with JavaScript. If that's the case then you could try a JS-based back-end. I have played with Meteor, which I can recommend.
rubyonrails.org
meteor.com
laravel.com