If you want to program even the basic functions that you will need on the internet today,
it becomes necessary to learn HTML, CSS, javascript & PHP.
HTML is used to organize and display your data.
CSS is used to format & organize your data.
Javascript is used to manipulate the objects and elements on the screen(it can also do anything that HTML and CSS can do),
and PHP is used for communicating with files & databases, manipulating arrays, sending email and a whole lot more.
So if you already have a basic understanding of HTML, than you are half way there to learning PHP.
PHP is used in the same document that your HTML is in.
You just need to learn how to talk to files with PHP.
The code I presented is the basic structure for creating and writing to a file.
The only other concideration for now is: HTML files usually have a file extention of .html or .htm, but when you add in PHP then you change the extention to .php that way the script interpreters know that you are programming in PHP.