When i think about it - one language wont do it. When you learn html/CSS - you will know how browser renders elements on the page, nothing else. If you learn javascript - you wont know anything about server side. If you learn PHP - it will complement your knowledge of html/css nicely, but you still need to know some javascript.

So my vote goes to PHP + javascript (+node.js) + HTML + CSS

edit: i recommend Ubuntu for development, or if you own a Mac, just install xampp and enjoy.