11
There are couple of deprecated notices when you use XOOPS 2.5.6 on PHP 5.5.x. It will work just fine, but you'll get couple of warnings about "deprecated function", but you can ignore them. If it bothers you, then update to PHP 5.4.x
If you want to use PHP 5.5.7, and it is not a "production server", you might update the XOOPS 2.5.6 you have to XOOPS 2.5.7 Beta. This one should work with PHP 5.5.7 without any problems.