or you can just include that HTML file into a php file.
file.html
<html>
<head>
</head>
<body>
<?php
echo "HELLO! <br />";
?>
</body>
</html>
file.php
<?php
include("file.html");
?>
Why can't you change the extension to php? Is it because PHP is not supported? If so you cannot execute the PHP at all.