PHP验证数据库密码问题

header("Content-type: text/html; charset=utf-8");class cls_Mysqli{    private $db_host="localhost";    private $db_user="root";     private $db_password="root";    private $db_name="test";    protected $sql;    public $password;        function __construct($id,$password){    	  $this->password = $password;    	  $this->sql = "select password from admin where id = $id";    }          function checkPassword(){    	  global $link;        $link = mysqli_connect($this->db_host, $this->db_user, $this->db_password, $this->db_name) or die("数据库错误: " . mysqli_connect_error());        $res  = mysqli_query($link,$this->sql);        if ($row_val = mysqli_fetch_row($res)){              if ($this->password == $row_val[0]){                  return true;              }else{                  return false;              }                      }    }}$query = new cls_Mysqli(1,'123');print $query->checkPassword();mysqli_close($link);