I upgraded today from UBB.classic to UBB.threads php so far so good but I have one huge issue. I need to include SSI output for our admanagent and I just can't get it to work and this is the first time using php.

HELP! What do I need to do execute a cgi ?

I've had the same problem while upgrading to v6. From PHP you can call the virtual() function to call old PERL scripts. Hope that helps. Unfortunately I can't use this function, because my provider doesn't support it

<?php virtual('/your/old.pl') ?>

It only works on Apache webservers, unfortunately.