[php-src] Issue #7930: serialzing xml

Issue: https://github.com/php/php-src/issues/7930 Comment Author: cmb69 SimpleXML is implemented on top of libxml2, and as such proper (un)serialization would require a lot of custom code to be implemented, and it seems unlikely that this would be considerably faster than (un)serialization from/to string. So use this instead, e.g. <https://3v4l.org/WqLfO>. If you feel strongly that proper (un)serialization should be implemented, please purse the [RFC process](https://wiki.php.net/rfc/howto).