/**     * 析构函数     */    //解决重复链接的问题    private $db_handler = null;    function __destruct() {        Log::logWrite($this->log_data, ‘db‘, ‘sql‘);        if(!is_null($this->db_handler[$this->key_back])){            $db_thread_id = $this->db_handler[$this->key_back]->thread_id;            $this->db_handler[$this->key_back]->kill($db_thread_id);            $this->db_handler[$this->key_back]->close();//断开数据库连接            unset($this->db_handler);        }    }    public function getDb(){        if(is_null($this->db_handler[$this->key_back])){            $this->db_handler[$this->key_back] = mysqli_connect($this->host, $this->username,$this->pwd,$this->databasename) or die("Could not connect: " . mysql_error() . "<br/>");            $this->db_handler[$this->key_back]->query("SET names $this->charset");        }        return $this->db_handler[$this->key_back];    }