i'll bet its available already for you.
just create a 1 liner program called : muhaha.php
<?php echo $_SERVER['PATH_INFO']; ?>
and upload to your root.. then point to it like
http://yourdomain.com/muhaha.phpit will --probably-- echo a unix-ie path like /home/bla/public_html or whatever..
if so, you can do the happy dance. then you just turn on spider friendlies in your cpanel and let it rip.
keep in mind that google can only spider your forums that are OPEN to guests as readable.. any private forums can't be spidered, since a login wouild be required
ps: phpinfo(); also tells you about this and MORE.. so choose your poison