Create a .htaccess file at the root of your website and add this line:
[Apache2 @ Ubuntu/Debian: use this directive]
AddType application/x-httpd-php .html .htm
Or, from comment below:
AddType application/x-httpd-php5 .html .htm
If your are running PHP as CGI (probably not the case), you should write instead:
AddHandler application/x-httpd-php .html .htm
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