public function addSchemaTypeSection()
    {
        if ($this->_schema === null) {
            $this->_schema = $this->_dom->createElement('xsd:schema');
            $this->_schema->setAttribute('targetNamespace', $this->_uri);
            $types = $this->_dom->createElement('types');
            $types->appendChild($this->_schema);
            $this->_wsdl->appendChild($types);
        }
        return $this;
    }