There are several methods:
index.php:
<?php
$_REQUEST['ubb'] = "portal";
include("/home/user/public_html/forums/ubbthreads.php");
?>.htaccess:
# Set Directory Indexing #
DirectoryIndex ubbthreads.php ultimatebb.php index.php maintenance.php index.shtml index.html index.htm
You could also setup an html redirect; many ways to do the same thing really. The index.php will make the forum load wherever the file is, the .htaccess will make ubbthreads.php an index document so if the directory is loaded it'll look for that as the index page.
