Description:
------------
$GLOBALS contains a reference to a magical array that has all the globals in it. Unsetting or assigning a value to it seems to forevermore lose the reference to this magical array, making $GLOBALS a regular old variable that has no connection to defined globals (although it's still a superglobal).

Unsetting or assigning to $GLOBALS probably should fail, just like $this does. I spent some time today trying to debug a scenario where $GLOBALS['someVar'] did not equal $someVar after I had written "global $someVar". It turned out to be because someone had unset $GLOBALS somewhere (they were trying to clear all global variables and restore them to an earlier state). This is really confusing and should be disallowed. Or at least produce a notice or something.

Test script:
---------------
$a = 1;
unset( $GLOBALS );
function f() {
  global $a;
  unset( $GLOBALS ); // $GLOBALS = []; is the same
  var_dump( $a );
  var_dump( isset( $GLOBALS['a' ) );
}
f();

Expected result:
----------------
Either:

int(1)
bool(true)

Or:

NULL
bool(false)

Actual result:
--------------
int(1)
bool(false)