oop编程 委托 和回调

/**
 * Created by PhpStorm.
 * User: Administrator
 * Date: 2021-10-10
 * Time: 19:26
 */
//被委托的类
class Query{
    //创建类的唯一实例 pdo对象
    private static $db;
    protected $table;
    protected $field;
    protected $limit;
  //private 私有 阻止此类在外部进行实例化
    private function  __construct(){
    }
   static function  connect($dsn,$username,$pwd){
        //创建PDO类的唯一实例 PDO对象
       if(is_null(static::$db)){
           static::$db= new PDO($dsn,$username,$pwd);
       }
       //返回query实例
       return new static();
   }
    public function  table($table){
         $this->table=$table;
         return $this;
   }
    public function  field($field){
        $this->field=$field;
        return $this;
    }
    public function  table($limit){
        $this->limit=$limit;
        return $this;
    }
    public function  getSql(){
        return sprintf('SELECT %s FROM  %s LIMIT %d ', $this->field, $this->table,$this->limit);
    }
    public function  select(){
        return static::$db->query($this->getSql())->fatchaAll(POD::FETCH_ASSOC);
    }
  class  Db{
   static function __callStatic($method,$args){
       $dsn='mysql:host=39.105.79.62;dbname=news';
       $usernmae='miujue';
       $pwd='zhoujielun521';
       //获取到被委托的类query实例
       $query=Query::connect($dsn,$username,$pwd);
       call_user_func([$query,$method],...$args);
  }
 }
   $res= DB::table('cate')->field('catname,catid')->limit(5)->select();
    echo '<pre>';
    print_r($res);