新しい mysqli でエラーが発生するのはなぜですか?

class db_mysqli extends mysqli<br />
 {<br />
 	<br />
 	private $link;<br />
 	private $result;<br />
 	private $bol=false;<br />
 	<br />
   function __construct(){<br />
 		$this->open_link();<br />
 	}<br />
 	<br />
 	//开启连接<br />
 	private function open_link(){<br />
 		$this->link=new mysqli($GLOBALS['HOST'],$GLOBALS['USER'],$GLOBALS['PASSWORD'],$GLOBALS['DBNAME']) or die("数据库连接失败！".mysqli_error());<br />
 		$this->link->select_db($GLOBALS['DBNAME']) or die("数据库选择失败！".mysqli_error());<br />
 		$this->link->set_charset("UTF-8");<br />
 	}<br />
 	<br />
 	//关闭连接<br />
 	private function close_result(){<br />
 		//if($this->result){<br />
 			//$this->result->free_sult();//关闭结果集<br />
 		//}<br />
 		<br />
 	}<br />
 	<br />
 	<br />
 	<br />
 	public function db_query($sqlstr){<br />
 		if(empty($this->link)){<br />
 			$this->open_link();<br />
 		   }<br />
 			$this->result=$this->link->query($sqlstr);<br />
 			if($this->result){<br />
 				$row=$this->result->fetch_array();<br />
 			}else {<br />
 				$row=null;<br />
 			}<br />
 			$this->close_result();<br />
 			return $row;<br />
 	}