Uhhh.... You DO know that you did it backwards, don't you? Your forums are running PHP3. Therefore, you should replace all PHP links with PHP3 links.
Search for:
- index.php
- register.php
- announcement.php
- attachment.php
- avatar.php
- calendar.php
- editpost.php
- forumdisplay.php
- global.php
- member2.php
- member.php
- memberlist.php
- misc.php
- moderator.php
- newreply.php
- newthread.php
- online.php
- poll.php
- postings.php
- printthread.php
- private2.php
- private.php
- report.php
- search.php
- sendtofriend.php
- showgroups.php
- showthread.php
- threadrate.php
- usercp.php
And replace with their filename, but "php3" instead of just "php". You have to make sure that you run PHP3 files though. If you run PHP instead of PHP3, reverse the instructions. (Search for *.php3 and replace with *.php)