By authenticating with the help of $PHP_AUTH_USER / $PHP_AUTH_PW, PHP has problems to set these variables when they include German Umlauts (???????). 

Here a list with correct and incorrect values:

$PHP_AUTH_USER:       result:
t?ten                        t?ten                (ok)
t?ten                        t?ten                (ok)
t?ten                        t?ten                (ok)
t?t?r                         t?t                  (error)
t?t?r                         t?t                  (error)
t?t?r                         t?t                  (error)
t?t?r?                       t?t                  (error)
t?t?r?                       t?t                  (error)
t?t?r?                       t?t                  (error)
da?                         da?                  (ok)
?ad                         '' (emtpy string)      (error)
T?R                        T?R                  (ok)
T?R?                      T?R                  (error)
?sterreich                '' (empty string)      (error)
?ste                        '' (empty string)      (error)
?st                         '' (empty string)      (error)
Tabalug?                Tabalug?             (ok)
Ta?_Kaff?               Ta?_Kaff?            (ok)
Ta?t?                     Ta?t?                (ok)
Ta??                      Ta?                  (error)
r?st?n                     r?st?n               (ok)
relo?m?                  relo?m               (error)

Testet with Apache 1.3.3 and PHP 3.0.5 on Linux and SUN Solaris. 
My do-conf line: 
./configure --with-apache=/usr/local/src/comapache/apache_1.3.3 --with-gd=/usr/local --with-mysql --with-config-file-path

This strange behaviour is discussed in the German PHP Mailing List (archive on http://infosoc.uni-koeln.de/archive/php/), but until now we haven't found a solution.