2
We do not have confirmation on PHP5 compatibility - we've made some attempts in a test version of XOOPS, but lack reliable feedback on what makes PHP5 choke on XOOPS and how these chokings can be remedied.
At first, we know that there are some methods which work differently as well as e.g. the clone() method on XoopsObject makes PHP5 choke because clone() is a new native method on all classes in PHP5.
If you want to help out, you can get this very experimental
XOOPS 2.0.8 (beta) and give us feedback on that one - just remember to explicitly state that it is this experimental version you are using when giving feedback.