How to rewrite .html url to. php file using .htaccess?

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HTML

PHP

Apache

I have changed a website from .html extensions to .php; the file names are the same, and there are thousands of files. There is php code now which cannot be parsed with .html

Basically, since now all the files have a .php extension, I want to rewrite the url of all html files to become .php

In other words, if someone visits:

mysite.com/page.html

then they are taken to:

mysite.com/page.php

Additionally, I need it to LOAD the .php page, not just change the URL.

How can I do this in .htaccess?

p.s. at last what appears to be a viable alternative to stackoverflow! Some a**hole took this question and said "duplicate question" and linked to an article about the basics of htaccess. They obviously have no idea what the word "duplicate" means! That generic page didn't even have anything remotely close to my question there! Not to mention there was another question that did the opposite that no one said was a duplicate!