PHP works on the Server Side (on the server hosting the website).
A single PHP file is like using magic on a webpage. In HTML, you have a standard page that is sent to anyone that visits your website. With PHP, you can dynamically change the page based on each individual user. Honestly, learn PHP and put it to the test. You will be extremely impressed with what an open source programming language can accomplish. I am not going to tell you that PHP is the best server side language that is out there, but it is the best for a tight budget or a beginner (Sorry, after coding with ColdFusion, I can no longer say PHP is my favorite).
What exactly does PHP do?
It can create custom content based on different variables
It is excellent at tracking user information
It can write or read information to databases, if partnered with a database language
It can run on any type of platform and servers
It can do anything a standard HTML file can… and much more