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