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   'localhost',        'port'=>'3306',        'user'=>'root',        'pwd'=>'mysql123',        'charset'=>'utf8',        'dbname'=>'test',    );    //私有，数据库连接资源id    private $link;    //静态实例化变量    private static $instance;    /**     * @param $params     */    //私有，初始化配置数组    private function initAttr($params)    {        $this->db_config = array_merge($this->db_config,$params);    }    /**     *     */    //私有，连接数据库    private function connectServer()    {        $host = $this->db_config['host'];        $port = $this->db_config['port'];        $user = $this->db_config['user'];        $pwd = $this->db_config['pwd'];        $dbname = $this->db_config['dbname'];        if ($link = mysqli_connect("$host:$port","$user","$pwd","$dbname"))        {            $this->link = $link;        }        else        {            die('数据库连接失败'.mysqli_error($this->link));        }    }    /**     *     */    //私有，设置数据库字符集    private function setCharset()    {        $sql = "set names {$this->db_config['charset']}";        $this->query($sql);        //mysqli_query($this->link,$sql);    }    /**     * @return bool     */    //私有，选择具体数据库    private function selectDefaultDb()    {        if($this->db_config['dbname'] == '')        {            return false;        }        $sql = "use {$this->db_config['dbname']}";        $this->query($sql);        //mysqli_query($this->link,$sql);    }    /**     * mysqldb constructor.     * @param array $params     *     */    //构造函数    private function __construct($params = array())    {        $this->initAttr($params);        $this->connectServer();        $this->setCharset();        $this->selectDefaultDb();    }    /**     * @param $sql     * @return bool|mysqli_result     */    //执行查询    public function query($sql)    {        if($result = mysqli_query($this->link,$sql))        {            return $result;        }        else        {            echo 'SQL执行失败：
';            echo '错误的SQL为'.$sql.'
';            echo '错误的代码为：',mysqli_errno($this->link),'
';            echo '错误的信息为：',mysqli_error($this->link),'
';            die();        }    }    /**     * @param $sql     * @return array|bool|null     */    //取单行数据    public function fetchRow($sql)    {        if($result = $this->query($sql))        {            $row = mysqli_fetch_array($result,MYSQLI_ASSOC);            return $row;        }        else        {            return false;        }    }    /**     * @param $sql     * @return array|bool     */    //取多行数据    public function fetchAll($sql)    {        if($result = $this->query($sql))        {            $rows = array();            while($row = mysqli_fetch_array($result,MYSQLI_ASSOC))            {                $rows[] = $row;            }            mysqli_free_result($result);            return $rows;        }        else        {            return false;        }    }    /**     * @param $data     * @return string     */    //转义字符，防止sql注入    public function escapeString($data)    {        return mysqli_real_escape_string($this->link,$data);    }    /**     * @param array $params     * @return mysqldb     */    //静态方法，获取唯一的数据库实例    public static function getInstance($params=array())    {        if(!self::$instance instanceof self)        {            self::$instance = new self($params);        }        return self::$instance;    }    //克隆类的方法    public function __clone()    {        die('禁止复制此类');    }}