When the client submits a file upload, the file he sends will be stored in the server's default temporary directory. You can set an alternative upload directory by altering the 'upload_tmp_dir' line in your php.ini file. The directory you use must exist and PHP must have write access to it.
However, if all you want to do is retrieve an uploaded file, you don't need to worry about where it is as the $FILES variable allows you to access the temporary version like this:
$_FILES['client_upload_name']['tmp_name']
You can use the move_uploaded_file function to move the file from the temporary location to somewhere else if you need to keep it.
