I configured index.php as my default document. Running appcmd list config /section:defaultDocument returns the same exact(order) list as in one of the previous post on this thread. My index.php is a simple helloworld - <?php echo'hello world' ?> . In my case it worked without issues.
Questions for you:
What happens if you configure something else as the default doc - let's say iisstart.htm, does it work ?
If you explicitly specify http://localhost/index.php does it work as expected ?
When you say the log file is clean, did you mean the w3svc log shows a status of 200 for GET / ?
Thanks,
Mukhtar Desai
IIS Performance Team