Description:
------------
Using simplexml fails with a warning like:

Warning: dom_import_simplexml(): Invalid Nodetype to import

I am using APC 3.1.10 and PHP 5.4.2. The issue does not appear when I downgrade to PHP 5.3.12.

This issue can easily be reproduced by running the example code found at http://www.php.net/manual/en/function.dom-import-simplexml.php

This breaks the symfony framework for example: https://github.com/symfony/symfony/issues/4185 (other distributions such as Fedora are affected as well)

Test script:
---------------
<?php

$sxe = simplexml_load_string('<books><book><title>blah</title></book></books>');

if ($sxe === false) {
    echo 'Error while parsing the document';
    exit;
}

$dom_sxe = dom_import_simplexml($sxe);
if (!$dom_sxe) {
    echo 'Error while converting XML';
    exit;
}

$dom = new DOMDocument('1.0');
$dom_sxe = $dom->importNode($dom_sxe, true);
$dom_sxe = $dom->appendChild($dom_sxe);

echo $dom->saveXML();

?>


Expected result:
----------------
<?xml version="1.0"?>
<books><book><title>blah</title></book></books>

Actual result:
--------------
Warning: dom_import_simplexml(): Invalid Nodetype to import in /home/pierre/public_html/testxml.php on line 10
Error while converting XML