Apache Handlers allow you to control how Apache will manage certain file types.
When Apache recognizes a file, it has an action built in for that file type and will perform that action. If you want Apache to perform a different action, you will need to override the default handler, to perform it.
For example, if you use a file type that requires a special service in order to run, such as a file with server-side includes that is not named with a .shtml extension, you need to tell Apache to treat these files differently.
Running HTML as PHP
However, if you wish to choose to continue, and use PHP for standard .htm, or .html files, you will be able to add the following line to your .htaccess file.
AddHandler application/x-httpd-php .html .htm
Please Note, this method may not work if you are running with the "PHP Selector" configured to use a non-native PHP version, as the handler may differ.