3
A PHP norice can't be the cause of your site not working, it's just a, well, notice really, saying that the coding wasn't done perfectly (in this case, a variable was used that wasn't defined beforehand).
Incama is right tho, PHP5 isn't officially supported by XOOPS, and there are some changes in PHP5 to PHP4 that can cause XOOPS not to function. We're looking into those tho, but untill then, PHP5 isn't supported.
Herko