variable file_directory_temp not set correct / Error: 'open_basedir restriction in effect' when uploading (solution included)

In an clean fresh Installation the variable 'file_directory_temp' is not set up correct in the script 'includes/file.inc' (function file_directory_temp).

Running Apache 2 with virtual hosts, PHP 4.3.10, Zend Engine v1.3.0, PHP.ini safe_mode=On + open_base_dir=/path/to/my/website + upload_tmp_dir=/path/to/my/website/php_tmp

If 'safe_mode = On' and 'open_base_dir = /srv/www/mysite.com/' is set in php.ini or via apache i get an error when trying to import an language:
open_basedir restriction in effect. '/tmp' is not within the allowed path(s): ...

The error seems to be in includes/file.inc line 687 (function file_directory_temp() ; see underlined line). It is in currently (starting at line 681 for better understanding):

// Has PHP been set with an upload_tmp_dir?
if (ini_get('upload_tmp_dir')) {
$directories[] = ini_get('upload_tmp_dir');
}

// Operating system specific dirs.
if (substr(PHP_OS, 0, 3) == 'WIN') {
$directories[] = 'c:\\windows\\temp';
$directories[] = 'c:\\winnt\\temp';
$path_delimiter = '\\';
}
else {
$directories[] = '/tmp';
$path_delimiter = '/';
}

The error should get fixed if changing line 687 to
elseif (substr(PHP_OS, 0, 3) == 'WIN') {
because if there is set up an upload_tmp_dir in php.ini, there should not be an check for Windows OS.
There should only be an check for Windows OS if no upload_tmp_dir ist set.

The error can only be reproduced when the variable file_directory_temp has not been set by drupal. The easiest way to get this condition should be to set up an new drupal installation.
I got little trouble to reproduce this failure due to some caching effects i think, or because in setup of drupal is set file_directory_temp up in an another script or so.

If you got further questions, please contact me.