Thank you Brion, you pointed me in right direction & I solved the problem. The session save folder was not writable by apache.

For the benefit of others ...

Info on php sessions: http://ca3.php.net/session

1) I verified that cookies worked on my client system. 2) On the server, I found out that no session files were being created in the folder pointed to by the "session.save_path" php variable. This is usually a permissions problem. 3) I tested this by making the session folder world writable. My sessions now work properly. 4) I determined the apache "user" & set up the permissions properly to avoid security problems.

...

Peter Work wrote: As an anonymous user, I can create new pages, edit & save articles just fine. If I log in as a user or even as WikiSysop, I don't seem to be able to write back to the database. The SAVE button in the edit window is treated like a PREVIEW, I cannot change prefences. As WikiSysop I cannot delete pages.

PHP session support is broken on your server. Check the session save path.

-- brion vibber (brion @ pobox.com)