PHP is where the clever stuff really starts to happen.  PHP is a programming language written in a similar way to JavaScript but which can create customised pages for you and your users. 

JavaScript can tweak a page after it exists but PHP can make a page for specific users with specific content aimed just at them which is different every time they look.

It can also transfer data in and out of databases but that will come later.  It is the language in which Facebook was first created and many other sites use it as well (look for the .php at the end of page file names).

This tutorial assume you know the basics of HTML (and some CSS).  You will also meet again many of the programming principles which were introduced in the JavaScript tutorial.  You need to be especially aware of HTML forms.