I installed today DokuWiki with no problems in my web site. However, my hosting service activated the PHP Safe Mode, and though I worked around the cache folder problem by creating manually those folders, I had to use the hack to allow for installing plugins.
But this still wouldn't work, even with the proper root path. I started poking around, and I got it to work by doing two small changes (see the attached diff):
* avoid using fullpath inside the preg_quote. It doesn't seem to work in my installation, which uses PHP 5.2.5. Actually, I haven't found it in the PHP5 docs. Where is it defined?
* change $tmpdir so it would be "$base/$dir" instead of just the concatenation, or no slash would be generated, getting something like "tmp<MD5 hash>" instead of the expected "tmp/<MD5 hash>".