I suggest to read this page of the tutorial. It seems, that it solves the questions, you posted here:
http://www.php.net/manual/en/configuration.file.per-user.php
Please be aware, that the ini-file is not re-read on every request, but after a defined time. Neither are all settings changeable in those per-user ini-files.
Read also the other pages in this chapter, they're good to keep in mind ;)
If you're now calling the script from a webserver, you called by requesting a page on a subdomain or a top-level-domain, doesn't matter.