Revert "Remove the PHP Global Scope object."

Although this isn't super-useful in one sense, it does make clear the
mirror-image aspect of the FFI.

This reverts commit 386131f350c2bce8edfb4e1557b9eb47908af778.
Make looking up an unknown name in the PHP global scope raise an exception.

Add tests even for the non-exception case, as this was not previously tested.