Description:
------------
The superglobals cannot get used as "variable-variables" 
in a function.

This is documented here: 
http://www.php.net/manual/en/language.variables.variable.php

Therefor this is a feature request, to change this.

Reproduce code:
---------------
$g = '_GET';
var_dump( $$g );

function f() {
  $g = "_GET";
  var_dump($$g);
}

f();


Expected result:
----------------
array(0) {
}
array(0) {
}


Actual result:
--------------
array(0) {
}

Notice: Undefined variable: _GET in X
NULL