Description:
------------
The attached testcase crashes PHP when run under ZS, and when run under Apache

Reproduce code:
---------------
<?php

$order_xsd = <<<EOF
<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ord="orderNS" xmlns:cust="customerNS" targetNamespace="orderNS">

  <include schemaLocation="./Customer.xsd"/>

  <element name="order" type="ord:OrderType">
    <complexType name="OrderType">
      <sequence>
        <element ref="cust:customer"/>
      </sequence>
    </complexType>
  </element>

</schema>
EOF;

file_put_contents('Order.xsd',$order_xsd);


$customer_xsd = <<<EOF2
<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
 xmlns:cust="customerNS" targetNamespace="customerNS">
  <element name="customer" type="cust:CustomerType">
    <complexType name="CustomerType">
      <sequence>
      </sequence>
    </complexType>
  </element>

</schema>
EOF2;

file_put_contents('Customer.xsd',$customer_xsd);


$xmldas = SDO_DAS_XML::create('Order.xsd');
$order = $xmldas->createDataObject('orderNS','OrderType');
$order->customer = null;

$o = clone $order;

?>