Here's a clue that indicates this is a mediawiki configuration issue, and not the php set up on my host.
We also have a Joomla installation that saves session data (for administrator access to the back end). That's working, and it's making files with session data in them in the same new folder and path I set up.
The Joomla site was also working before I made this path change too. No doubt before the path change the mediawiki session files were being written into the servers /tmp folder, but they were empty. The only change is this is all happening in my server area where I can see what's going on.
So, why would mediwiki be making session files and not writing to them while Joomla can, even though it's using the same php.ini settings?
Any help with this would be greatly appreciated. Thanks, Scott