数据库查询构造器

作业问题:

代码：

<?php
class Query
{
    //创建类的唯一实例
    private static $db;
    protected $table;
    protected $field;
    protected $limit;
    private function __construct()
    {
    }
    static function connect($dsn,$username,$pwd)
    {
        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;
//        echo $this->table;
        return $this;
    }
    public function field($field)
    {
        $this->field = $field;
//        echo $this->field;
        return $this;
    }
    public function limit($limit)
    {
        $this->limit = $limit;
        return $this;
    }
    public function getSql()
    {
        return 'SELECT '.$this->field.'  FROM '.$this->table.' LIMIT '.$this->limit;
//        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, $arygs)
    {
        $dsn = 'mysql:host=localhost;dbname=test_db';
        $username = 'test_db';
        $pwd = '123456';
        //获取委托实例
        $query = Query::connect($dsn,$username,$pwd);
        return call_user_func([$query,$method],...$arygs);
    }
}
//$res = Db::table('t_user')
//    ->field('name')
//    ->limit(1)
//    ->select();
//
//print_r($res);
print_r(Db::filed('nihao'));