You could add the following code to your Apache config file inside of <IfModule mime_module> tag:
AddType application/x-httpd-php5 .html .htm
If you need to support other file types, like “..xml”, simply add them to the list, like this:
AddType application/x-httpd-php5 .html.xml
The entire text should look like this.
AddType application/x-httpd-php .php
Also, you could add the same line of code, anywhere inside your .htaccess file.