dont really understand this question..
If the web server has PHP installed then you would have the .php extition for php files and then you would configure you web server to execute .htm .html .php files as default extitions. You would also make the index scripts index.htm index.html and index.php
Whats the problem?
Just right a page or the main page and save it with a .php extition.
just write the page that has your php at the top of the page then the rest would be your html page in the .php file..
<?
echo "This is a test";
?>
<html>
<body>
this is the html main page
</body>
</html>