It won't necessarily be hidden - it would if the server isn't yours, of course, because fiddling with it would affect everyone else on that server who uses PHP.
If the web server is Apache, PHP is compiled in as a module, and you have appropriate permissions to do so, you can also make some adjustements with an http.conf file in the directory in question. http://nz2.php.net/manual/en/configuration.changes.php
But if you do have the right to fiddle with it, you can find it using phpinfo().