5
No, Php5 should already be supported, that's all.
Taking late with support for Php 5 is giving late to Xoops, one more time.
When I see the time required to release a version (when it is released) I can imagine that the 2.5 will be released in, at least, 1 year.
Xoops will be, one more time, the last project to support something, in this case Php 5.