php将表导成xml格式

public function structureDirectoryXml(){
        $strXml='';
        if(!empty($_GET['id'])){
        $this->load->dbutil();
        $directory=$this->db->query('select * from course_directory where course_id='.$_GET['id']);
        $config = array (
                  'root'    => 'directory',
                  'element' => 'item',
                  'newline' => "\n",
                  'tab'    => "\t"
                );
        }
        $strXml.=$this->dbutil->xml_from_result($directory,$config);
        
        return $strXml;
    }