What is PHP?
HTML allows to choose the layout of the web page and it's not a progamming language.
Viceversa PHP is a programming language, like pascal or C++.
A programming language produces dinamic and/or interactive pages and their contents can change depending on the user's choices.
You can include a PHP code into a HTML code by using the tags:
A programming language code included in a web page is named script.
You can edit a PHP program as a HTML page using a textual editor (i.e. HTML-kit), but you must save it with .php extension