Registered users cannot make modifications in MediaWiki

This was bugging me for a while. There was a server restart and after that no registered users could make any changes in our MediaWiki install - unregistered users could write, change, whatever without any problems. I've updated to 1.4.5, no help. Finally I've turned to the IRC channel of MediaWiki - and a simple answer led me to the solution: "your php session support is broken. fix it."

I've checked my php.ini, and there I found:

session.save_path = /tmp/php4_sessions

Thinking a bit back I remember adding this subdirectory to the config a while ago, when I had enough of all the session files in the /tmp folder - what I forgot is that at restart, the contents of the /tmp folder is completely deleted - including subdirectories. Also, php warns me at the config that she'll not create subdirs for the sessions, so there it is - I've changed back the php.ini config to /tmp, and now everything works well.