A server side language
PHP is a server side programming language.
This means that the PHP code must be interpreted and executed by a specific PHP module residing in the server.
When you write a PHP program, you can see it executed in the web only, not in your computer, unless you installed in it the PHP module as well as the program that transforms your PC in a server.
In order to see your program executed, you cannot use the preview of HTML-kit (it interprets HTML only) or click on the file name, as you usually do with HTML, text or image file.
In order to execute a PHP program you must publish your page in the server and search for it by its URL in Internet.
How it works:
The page you are seeing is a .php page (look at the URL).
If you tries to display the source code of .php page, you'll see the HTML code only.