PHP is a drug!
It's also a server side scripting engine that allows you to run scripts (programs) on the server side of an HTTP transaction.
The '.php' extension tells a php language parser on the web server to parse and process instructions in this file. The parsed file then gets kicked back to the web server which, if all goes right, gets thrown back to the requesting client -a browser usually.
HTML, on the other hand, gets parsed in your browser. The .html file just sits on the web server and waits for someone to request it. Nothing special happens as it sits there all alone waiting for some far off, or even local, web browser on a client requesting machine to invite it over. Once it walks over to your browser it starts to become happy and talks to your browsers HTML parser.
In short, .html files gets parsed by your browser while .php files get parsed by a PHP friendly machine.
What are you trying to accomplsh?