Description:
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From the implementation I suppose this is pretty obvious, but the following does not work:

   $data_source = '_POST';
   $data =& $$data_source;

'$data' does not point at $_POST as expected. I'd consider this a problem - your implementation is showing through. Either re-label $_POST, etc. as not actually being superglobals, or fix the underlying implementation (which does not at present match up with your documentation).

"""This is a 'superglobal', or automatic global, variable. This simply means that it is available in all scopes throughout a script."""

This statement is not true, since the variable is only 'automatic global' when referred to by name directly (and thus gets caught by your special case code). It does not actually exist in every scope.

I beg you not to mark this as a documentation problem and change the docs to mention this. Fix it! It's a horrible inconsistancy. This, along with a million other sour points, is the reason why I absolutely detest my day job at the minute. Come on guys! Do it for me!

Reproduce code:
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$data_source = '_POST';
$data =& $$data_source;

echo gettype($data);

Expected result:
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array

Actual result:
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NULL