数据库连接及事件委托

<?php
/**
 * 事件委托：数据库查询构造器
 */
//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 $limit;
    //private 私有的  阻止此类在外部进行实例化
    private function __construct()
    {
    }
    static function connect($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 $this;
    }
    public function limit($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())->fetchAll(PDO::FETCH_ASSOC);
    }
}
class Db
{
    static function __callStatic($method,$args)
    {
        $dsn = 'mysql:localhost;dbname=yu';
        $username = 'root';
        $pwd ='123456yu';
        //获取到被委托的类query实例
        $query = Query::connect($dsn,$username,$pwd);
        return call_user_func([$query,$method],...$args);
    }
}
$res = Db::table('cate')->field('catname,catid')->limit(5)->select();
echo '<pre>';
print_r($res);
?>