If you want to run php code in your html web pages we recommend that you rename them using the ".php" suffix and run php natively, i.e. index.php rather than index.html.
However, if you cannot or do not wish to rename your html pages you can add the following code to your .htaccess file:
AddHandler application/x-httpd-php5 .html .htm
This will then force any pages in your site with a ".html" or ".htm" extension to be treated as though they had a ".php" extension.
We recommended that you never allow html pages to automatically handle php or shtml, because this forces all of your html pages to be processed by the server first.