6
I was hoping, it would be PHP4x, as some users reported some issues there. I had some problems on PHP5.25, but after I made sure that all the xoops_data and xoops_lib directories were set to 777, the issues went away. Could you try to reinstall it, but during the installation make sure that the directories are indeed set to 777.
Remember, this is still "alpha" release, so not everything will work as intended, and we need all the feedback from test users.