Hi,
might be some kind of an ugly solution but might work. AFAIK you can put AddType directive into .htaccess files if the web server is running apache and AllowOverride is set to on in httpd.conf. Try to put a .htaccess into DOCUMENT_ROOT that contains:
AddType application/x-httpd-php .html
AddType application/x-httpd-php .htm
That should make apache to interpret .html and .htm files as PHP files, too.
A javascript solution:
<html>
<head>
<title>Test</title>
<script>
url=document.location.toString()
url=url.replace(/(file.html)/,'file.php')
document.location=url
</script>
</head>
<body>
</body>
</html>
Thomas