Prevent SimpleXML from silently modifying types of variables that are

Prevent SimpleXML from silently modifying types of variables that are
assigned to its objects.

Implementation notes for overloaded object modules:

- If you return a zval which is not otherwise referenced by the extension
  or the engine's symbol table, its reference count should be 0.
- If you receive a value zval in write_property/write_dimension, you
  may only modify it if its reference count is 1.  Otherwise, you must
  create a copy of that zval before making any changes.  You should NOT
  modify the reference count of the value passed to you.

Have fun!