sys_get_temp_dir in shared hosting environment

Note: This could also fit in superuser.

I am setting up PHP 5.3.10 on a shared host with apache2 mpm itk and open_basedir in a way, that each user may not see or change the files of another user. In the apache2 vhost settings, I add the appropriate entries to restrict the user:

    AssignUserId     userA userA
    php_admin_value  open_basedir      /home/userA/www/
    php_admin_value  upload_tmp_dir    /home/userA/www/tmp/
    php_admin_value  session.save_path /home/userA/www/tmp/
    SetEnv           TMPDIR            /home/userA/www/tmp/

Now, the first line sets the linux user to use for apache2, the next three lines define the basedir, upload directory and session savepath to be in the user directory. I'll get back to the last line in a sec.

Now for the problem: sys_get_temp_dir() should give back the temporary directory for php, which is /tmp be default on a linux system. For security reasons, this directory should reside in the open_basedir of userA. According to the php-source of 5.3.10, the sys_get_temp_dir()-function uses the environment variable TMPDIR to get this directory:

     // php-src/main/php_open_temporary_file.c:217-219
     /* On Unix use the (usual) TMPDIR environment variable. */
     {
             char* s = getenv("TMPDIR");

This is what the fifth line in the configuration above should do. However, sys_get_temp_dir() simply returns the global system directory, ignoring the environmental variable (which is perfectly set in $_SERVER, also viewable via phpinfo()).

This results in some nasty bugs with various software relying on sys_get_temp_dir(), as that directory is outside of the open_basedir setting. I've tried to set the variable directly into $_ENV and $_SERVER without a change in behaviour. I've tried a putenv('TMPDIR=/home/userA/www/tmp') wit