Assuming you've got xampp installed in the root of C (the default location)...
I wouldn't really recommend it as it adds an unnecessary overhead on actual HTML files but you can do it ... in C:/xampp/apache/conf/extra/httpd-xampp.conf
you should see something that looks like:
LoadFile "C:/xampp/php/php5ts.dll"
LoadModule php5_module "C:/xampp/php/php5apache2_4.dll"
<FilesMatch "\.php$">
SetHandler application/x-httpd-php
</FilesMatch>
<FilesMatch "\.phps$">
SetHandler application/x-httpd-php-source
</FilesMatch>
You can tell Apache to treat .html files as PHP by adding a new FilesMatch directive:
<FilesMatch "\.html$">
SetHandler application/x-httpd-php
</FilesMatch>
Or you can do it by adding an .htaccess
file into your document root that contains something like AddHandler application/x-httpd-php .html
There are much better ways to do it though, use RESTful URLs for instance (and thereby avoid having the file-type even hinted at) and keep all your PHP files as .php
- but you'll probably need to get into the dark arts of Apache mod_rewrite for that.