IIS 7 and Above
Feb 10, 2009 02:43 PM|ruslany|LINK
It looks like Joomla tries to check folder "/tmp" at the file system root. Since the open_basedir does not allow access to that location the warning occurs. The easiest way to fix that I think is to add the "C:\tmp" to the open_basedir, e.g.:
open_basedir = \\hb1411\iisstorage\web;c:\windows\temp;c:\tmp
Note, that you do not actually have to have this directory on a file system as Joomla will fall back to configured session directory if it cannot write to tmp folder.