Apache - Configure - Configure Apache to treat files with HTML extensions as if they are PHP files

Create (or edit) the .htaccess file.

AddType application/x-httpd-php .html

NOTE: This setup can also be added to the main Apache configuration file, httpd.conf.

The .htaccess file can be further customized to only make the configuration effective to one PHP file.

To setup only a limited number of html files, use the following format:

<files filename.html>
AddType application/x-httpd-php .html
</files>

NOTE: The <files>, <filesMatch>, and <location> directives can be used.

A .htaccess file can be created in any directory within the document root, and when present the containing folder and sub-folders will inherit the configuration defined in the .htaccess file.
Another .htaccess file can be created in a sub-folder, and it will override the configuration defined by the parent configuration.

NOTE: One downside of setting up a web server to use html extensions for PHP scripts is performance.

Every HTML file will run through the PHP parser, and it will have a minor performance impact which will not likely be noticeable.