For whatever reason, our corporate TWiki requires exactly 3 spaces for bullets formatting. (doc says "at least 3 spaces", but it didn't work for me with 4 spaces).
Personally, I find that a bit overboard for my pursposes. Though to be fair, it could a great prep for publishing fanbase material. On the other hand, do any of us really want to post our own gotcha's by avid canon nerds?
I use many text files, and xmind, now version 8, mind map freeware.
IMO some sort of wiki would suffice quiet well for this. http://twiki.org/ also they have a ready to go vm here http://twiki.org/cgi-bin/view/Codev/DownloadTWikiVM or DukuWiki come to mind duku is as simple as installing ubuntu server and apt-get install dukuwiki
I use and am very happy with TWiki. It has authentication for editing, but you might need to add some other wrapper around it (like Apache authentication) if you want to restrict viewing. Has revision control and a WYSIWYG editor. It fails a few of your criteria though - it's in perl, not PHP, and it's not database-backed. The perl part never bothered me because I never found the need to hack on it. I really like the data backend however. Each page is actually stored as a plain text file on the server and versioned with RCS. This is great because even if your web server is totally boned, as long as you have access to the filesystem you can easily read the text files. This also makes the data very portable if you want to move to a different system in the future.