[PHP] PHP and empty() if form value is 0

[Phil Powell]

foreach ($HTTP_GET_VARS as $key => $val) {
   if (!empty($HTTP_GET_VARS["$key"])) ${"$key"} = $HTTP_GET_VARS["$key"];
  }

  foreach ($HTTP_POST_VARS as $key => $val) {
   if (!empty($HTTP_POST_VARS["$key"])) ${"$key"} = $HTTP_POST_VARS["$key"];
  }

Whenever the form variable is equal to 0, the value is not passed into the 
corresponding variable.

For example, if isLogin = 1, then $isLogin = 1
But if isRegistered = 0 then $isRegistered = {null}

I do not understand why this happens, someone enlighten me!

Phil