I found a wonderful answer right here in phpBuilder.
FrontPage creates a file called .htaccess this file can be used for many things. security, etc...
One is how to parse certain files. Add this line to the end of the .htaccess file in your main directory.
AddType application/x-httpd-php .htm .html
The information I found was here: http://www.phpbuilder.com/tips/item.php?id=79
Make sure that you first back up the .htaccess file just incase.
You can now keep your file named *.htm and still have it parse the php code.
One more thing.
To keep the php code in FrontPage from getting all screwed up, re-aranged, etc.. add this before and after your php code.
<%
<?
place php code here
?>
%>
It will trick FrontPage into thinking you have asp code and leave everything formatted as is.