Files will, by default be stored in the server’s default temporary directory, unless another location has been given with the upload_tmp_dir directive in php.ini. The server’s default directory can be changed by setting the environment variable TMPDIR in the environment in which PHP runs. Setting it using putenv() from within a PHP script will not work. This environment variable can also be used to make sure that other operations are working on uploaded files, as well.
It must be possible to output $TMPDIR to see what it is at the moment. Or you could overrule what it is with the upload_tmp_dir in php.ini so you know for sure where it is. Or did you read this already and do I not understnad you?