To be able to handle php script in .html file , you need to add the following in the .htaccess file:
AddType application/x-httpd-php5.html
To check that the setup works you need to create the following .html file and add the following php code in it:
<html>
<head> </ head>
<body>
<? php echo "Hello world!";?>
</ body>
</ html>
Save the file named test.html and upload it to your web hosting account.
When writing the address http://yourdomain/test.html in the browser you should see the message Hello World!