如何使用SimpleXML框架处理继承？

<xs:complexType name="BaseType">
  <xs:sequence>
    <xs:element name="baseElement" type="xs:string"/>
  </xs:sequence>
</xs:complexType>

<xs:complexType name="DerivedType">
  <xs:complexContent>
    <xs:extension base="BaseType">
      <xs:sequence>
        <xs:element name="derivedElement" type="xs:string"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

$xml = <<<XML
<DerivedType>
  <baseElement>Base element</baseElement>
  <derivedElement>Derived element</derivedElement>
</DerivedType>
XML;

$derivedType = new SimpleXMLElement($xml);
echo $derivedType->baseElement; // 输出 "Base element"
echo $derivedType->derivedElement; // 输出 "Derived element"

$elements = $derivedType->xpath('//baseElement'); // 查询所有baseElement元素
foreach ($elements as $element) {
  echo $element;
}