How do I add PHP code/file to HTML(.html) files?

In order to use php in .html files, you must associate them with your PHP processor in your HTTP server's config file. In Apache, that looks like this:

AddHandler application/x-httpd-php .html

I think writing PHP into an .html file is confusing and anti-natural. Why would you do that??

Anyway, if what you want is to execute PHP files and show them as .html in the address bar, an easiest solution would be using .php as normal, and write a rule in your .htaccess like this:

RewriteRule ^([^.]+)\.html$ $1.php [L]

You can't run PHP in .html files because the server does not recognize that as a valid PHP extension unless you tell it to. To do this you need to create a .htaccess file in your root web directory and add this line to it:

AddType application/x-httpd-php .htm .html

This will tell Apache to process files with a .htm or .html file extension as PHP files.