I was having numerous problems with modules when I installed XOOPS on a new FC4 install.

XOOPS Version: 2.0.13
Module Name/Version: Headlines, Forums, you name it...
PHP Version: 5 initially, now 4.3
MySQL Version: 4
Web Server Software: Apache
Operating System: FC4

Some of the errors I encountered included:
*not being able to edit any headlines - I could not add, edit,
remove any headlines
*multiple php errors including "already defined variables",
"undefined variable: op in file modules"
*blank pages with the XOOPS tag only
*couldnt add forums
*a bunch more

After messing around with trying to install and configure different XOOPS versions, I finally downgraded php5 to php4 and everything is working!.

So my fix for all the trouble I was having with XOOPS modules related to running php5. As soon as I removed php5 and installed php4 -- all modules are working like they should.

Here is an article on installing php4 on fc4: http://www.mjmwired.net/resources/mjm-php4-fc4.html

Are there known compatibility issues with php5 than can be fixed be editing the php.ini file? I'm happy that everythign is working now, but I'm interested if is something simple to get all the modules running on php5.

thanks.
-mat