Clash with mbstring extension

That's because mbstring doesn't like empty arguments.
The fix is described here (in russian), thanks to Basielienis: http://drupal.ru/node/view/310
Shortly, you should search in menu.inc (line 910 in 4.5.0) and replace line

$parent = substr($parent, 0, strrpos($parent, '/'));

with

if ($parent) $_lenght = strrpos($parent, "/");
$parent = substr($parent, 0, $_lenght);

ps: Can anybody commit this in HEAD?

This looks a bit strange to me (I haven't tested it though). Isn't $_length uninitialized if $parent is non-empty?Is that desired? If yes, though it may work, it's not a very obvious or clean solution, IMHO.

Yes, but disabling function overload you disable using unicode aware string functions in php.
As a result, you loose case insensetive searching in languages other than English.
AFAIR, drupal doesn't use mbstring extension directly, so the only way for non-english speaking people to use full functional searching is to enable function overloading, so strrpos() is automagically replaced with mb_strrpos(), ereg() with mb_ereg() etc.
I hope that one day somebody of developers will fix this annoying bug..

This won't be fixed unless there is a clear description of the problem and its solution, which complies with the Drupal coding standards.

Well, I am competely new to Drupal, and this error was part of my first impression. It should be fixed as mbstring extension should not need to interfere with Drupal and the problem is there in 4.6.0 as well. My host has this extension on by default.

As far as I can understand, strrpos gets replaced with mb_strrpos by the mbstring extension (http://php.net/manual/en/ref.mbstring.php) and mb_strrpos doesn't like the empty haystack unlike strrpos (at least in php4).

So the current code (line 973-976) in version 4.6.0 is

      do {
        $parent = substr($parent, 0, strrpos($parent, '/'));
      }
      while ($parent && !array_key_exists($parent, $_menu['path index']));

and could be replaced with

      do {
        $parent = substr($parent, 0, strrpos($parent, '/'));
      }
      while (($parent || $parent!="") && !array_key_exists($parent, $_menu['path index']));

There might be better and more logical solutions to this problem, but this at least seams to fix it for me.

Drupal 4.7 has a unicode layer which deals with mbstring. Note however that function overloading is optional for mbstring, and explicitly unsupported in Drupal. Drupal now instructs you in how to fix a broken mbstring setup.

This problem makes the theme settings unusable if mb_strrpos is active and I think it should be fixed in all supported versions of Drupal. Including HEAD because even if mb_string overloading is enabled that is no excuse for throwing these warnings.

The solution is to wrap the do..while in an if to make sure the loop body is not executed if $item['path'] is empty. Which happens every time the menu tree is built when root is processed.

Here's a patch which applies with various offsets against all Drupals from 4.5 to HEAD.

I wonder why $item['path'] is empty to begin with ... is that the correct/expected behavior?

Menu item 0 always has a blank path. So one warning is thrown every time the menu is built.

Committed to HEAD, DRUPAL-4-6 and DRUPAL-4-5. Thanks.

to make things short: it appears to have reappeared in 4.7.3. Still the requirements are in conflict to those of egroupware (and probably phpgroupware as well).

Why is it so hard to fix this permanently?

best wishes

Beate

please excuse, i just noticed that the problem seems to be a missing fix in the taxonomy_menu module. So i reported it in the wrong place.