1.php

<?php
session_start();
$xml = new SimpleXMLElement('<root />');
$_SESSION['xml'] = $xml;

// Fatal error: Uncaught exception 'Exception' with message 'Serialization of 'SimpleXMLElement' is not allowed' in [no active file]:0
// Stack trace:
// #0 {main}
//  thrown in [no active file] on line 0

2.php

<?php
session_start();
$xml = json_decode( json_encode( new SimpleXMLElement('<root />') ) );
$_SESSION['xml'] = $xml;