PHP - Crear un complexType abstracto con nusoap

<xsd:complexType abstract="true" name="abstractClass">

  <xsd:sequence>

    <xsd:element minOccurs="0" name="identifier" nillable="true" type="xsd:string"/>

  </xsd:sequence>

</xsd:complexType>

$server->wsdl->addComplexType(  'abstractClass',

                                'complexType',

                                'struct',

                                'sequence',

                                '',

                                array('identifier'  => array('minOccurs' => '0',

                                  'name' => 'identifier','type' => 'xsd:string'))

);