I don't understant your problem.
Currently, your portal is displayed on
www.your_domain/forum/portal.php
You want to display your portal at this adress :
www.your_domain/portal.php ?
In this case, you must upload portal.php file directly in your public directory. (
www.domain.com/ ) and you must edit the portal.php file indicating the chdir
/home/domain/public_html/