You can use an Apache directive in an .htaccess file to either parse all documents for .shtml or you can use the so-called 'XBitHack' (do a search on google or another search engine to get more information on how to use this) - basically you either set your html files to be executable or not depending on whether you want them to be parsed for Server Side Includes or not.
If you want to parse html files for PHP, you'd add something like this to your .htaccess file:
AddType application/x-httpd-php .php .php4 .php3 .phtml .txt .html .htm
This is an example; in this example, files with the extensions .php .php4 .php3 .phtml .txt .html .htm are all parsed for PHP.
If you have any other questions, feel free to contact support, or check the documentation at: