php 类中调用另一个类

require_once("mysqli.php");

$db = new db_mysqli();

class PayService
{
public $db;

public function __construct(){
$this->db = $GLOBALS["db"];
}

//随机生成站内测试订单号
public function getOrderNo(){

$query = "select * from ne_goods_pic;";

$data = $this->db->select($query);
return $data;
}

}