The only way you’ll get that to work is to change your web server to send .html files to the PHP parser. By default .html files are processed the same as .htm files.
I feel it’s a good point to follow the chain of operation here (as i understand it.);
When you call to www.yourserver.com/index.php;
The Webserver (most likely Apache, so i’ll call it that) receives the request.
Apache sees an extension of .php; in it’s config files, it knows that .php files need to be sent into the PHP module to be processed first (hence, PHP: Hypertext Pre-processor. Dont ask me why it’s a recursive acronym.). So it sends it there.
The PHP component kicks in, and parses all RELEVANT code segments (things encased in <?php ?>, or if you have short tags, <? ?>, etc.). It then sends the fully formed HTML back to Apache.
Apache receives the HTML, and kicks it out to the browser, which then interprets it into the webpage display.
The PHP component ignores everything outside it’s own tags - which means it’s fully acceptable to have HTML, JS, CSS, etc in a “.php” file. Which is why simply renaming your file will work.