Are there any forum softwares in PHP5?

Pretty much all PHP based forum software supports PHP5 now.

As bear says, all of them basically.

Can you clarify a bit more? Some work fine with PHP 6 too (SMF does, apart from one or two notices; and PHP 6 is still massively a WIP).

You can try phpBB or vbulletin

MyBB, PHPBB, SMF, and this Wikipedia article.

http://en.wikipedia.org/wiki/Comparison_of_Internet_forum_software_%28PHP%29

Phorum and AEF if you want to try something new and better (in my opinion).

You can try using the PhpBB its open source world leading forum software.

vBulletin 3.8.4 has PHP5 Capabilities, with plenty of recoding taken place to update to the new PHP.

http://vanillaforums.org/ is PHP5 only.

They have recently released v2 that's built around their own PHP5 framework called Garden. It's still early in development but looks like a promising architecture that's based on a plug-in/module system.