The most security conscious hosts turn PHP's Register Globals directive OFF by default. The next best allow you to turn it off in local .htaccess or php.ini files. A host that requires you to run a site with Register Globals ON should be avoided.
This is true for any PHP enabled site, whether or not you are running Joomla!. There is a legitimate argument to be made by hosts for keeping Register Globals ON for PHP4 sites. This is that it would break too much legacy code. This argument should not be accepted for a PHP5 installation. Beginning with PHP5, the official PHP recommendation was to keep Register Globals is OFF. Note that beginning with PHP6, there will not even be a Register Globals setting, so don't get caught in a Register Globals backwater. Modify your code to work without Register Globals, and choose a host that encourages such practices.
