[php-src] Issue #14742: SimpleXMLElement Serialization

Issue: https://github.com/php/php-src/issues/14742 Author: bchubb-web ### Description From what i can tell the SimpleXMLElement class can be very easily serialised/unserialised which would be very useful for use with var_exports and keeping state between requests without the need for the stored source for the xml, would it be possible to have __serialise() which outputs the string representation of the xml, and __unserialise() which just calls simplexml_load_string() and returns the new instance? the __set_state() method would also be useful but serialise would be beneficial to a larger audience