Process/Parse PHP from HTML files using .htaccess and allowoverride -
Process/Parse PHP from HTML files using .htaccess and allowoverride
RemoveHandler .html .htm
AddType application/x-httpd-php .php .htm .html
Just add the above into the .htaccess file for your website.
Also remember that you need to be allowed to override the Apache and this should go into the vhost for your site as shown below:
# you need the AllowOverride otherwise .htaccess directives will be ignored