While PHP variables obligingly spring into existence the first time you use them, relying on this behavior is a bad idea for two reasons. First, relying on the default value of an uninitialized variable can cause problems in some cases. Second, and more importantly, it can pose a security risk when ``++register_globals++`` is enabled. (Note that ``++register_globals++`` is deprecated in PHP 5.3 and removed in PHP 5.4.)