There is a tool to go from phpBB to MyBB RC4 (ot available for PR2 yet), but if you want a MyBB to phpBB convertor, your asking the wrong people. Go to
http://www.phpbb.com/
What about IPB2->MyBB? Or
WBB2->MyBB?
Nup, but a global converter script is/was being made by Chris Boulton. Once its finished, hopefully more convertors will be available.