Thread:Project:Support desk/WebStart.php issues/reply

Sounds like __DIR__ is returning something dirname doesn't recognize:

Relavent code: chdir( dirname( __DIR__ ) ); require dirname( __DIR__ ). '/includes/WebStart.php';

So if dirname returns '.' as your error indicates, and php docs say it can in certain situations (but they're not specific what situations) then MediaWiki won't access things in the parent directory properly, and the installer won't work.