The vast majority of UBB implementations are version 5, which is a PERL script. UBB6 has a PHP wrapper, I think, but is not an PHP script. In any event, the tepid response to UBB6 probably prompted Infopop to start thinking about a PHP board, as they were losing many potential upgraders to vBulletin. Scream, the W3T programmer, noted when the acquistion was being discussed, that almost all his new licenses were for the PHP version.
Eh, what others? :)
samantha