Thread:Project:Support desk/What does "writable by PHP" mean?

I've started getting error messages complaining about permissions, eg "The directory used to store the settings, /home/myuser/public_html/serialized/, is not writable. Please make it writable by PHP to use this extension."

I log in as "myuser" and am able to:

$ ls -ald /home/myuser/public_html/serialized/ drwxr-xr-x 2 myuser myuser 4096 Sep 12 2010 /home/myuser/public_html/serialized/

So clearly "writable by PHP" means something other than its normal meaning --

either that or my web server has started executing under another user. Is there a way to determine the name of that user by looking at MediaWiki's special pages that I can find -- or is there?