sys_get_temp_dir in shared hosting environment
Running a putenv('TMPDIR=/foo/bar') inside PHP seems to be able to affect the result of sys_get_temp_dir(). You could have an auto_prepend_file directive arranged to run a piece of PHP to set up the TMPDIR and avoid messing with a redefinition of sys_get_temp_dir().
Edit: Also, you could easily use putenv('TMPDIR='.ini_get('open_basedir').'/tmp') to set the temporary directory to the directory structure you laid out in the question.
Funny enough, this turns out to also work (given that you keep the SetEnv TMPDIR /foo/bar in your Apache configuration):
putenv('TMPDIR='.getenv('TMPDIR'));
Seems like a no-op, but actually does have effect on sys_get_temp_dir(). I'm starting to suspect this has to be some environment-handling bug in PHP.
You have tagged your question cgi, however you are making use of
php_admin_value open_basedir /home/userA/www/
^^^^^^^^^^^^^^^
which is a setting for the apache module version of PHP, Mod_PHP. In that case PHP is loaded once the webserver starts.
Then you make use of SetEnv:
SetEnv TMPDIR /home/userA/www/tmp/
this is setting an internal environment variable. It is passed to other apache modules, however I think you need it with the request, not with the virtual server. I don't know it specifically, but I would assume according to your description that this environment variable is getting reset before the PHP script is invoked.
So more a comment than a real answer, but hopefully it helps you clarify some things.
I normally use FCGI for multi-user environments so that I can better separate the users. I never had problems with setting environment variables per each user. But that's just another comment, I don't want to say you have to use it, too. Just to highlight that you need to find out the right place within apache to set the environment variable so it is (still) set when the script is executed.
Also you might not be setting the right environment variable. According to Apache Documentation about environment variables:
Although these variables are referred to as environment variables, they are not the same as the environment variables controlled by the underlying operating system. Instead, these variables are stored and manipulated in an internal Apache structure. They only become actual operating system environment variables when they are provided to CGI scripts and Server Side Include scripts. If you wish to manipulate the operating system environment under which the server itself runs, you must use the standard environment manipulation mechanisms provided by your operating system shell.
You want to set the operating system environment variable for PHP. But you are setting the internal environment variable only.
Mod_PHP might import them to the script, so if you use getenv('TMPDIR') the PHP SAPI specific implementation is used - which does allow you to see those internal environment variables - however the php_get_temporary_directory function is not using it - it looks like.
Please add your Apache and PHP version to your question.