<?php
  namespace org;
  /**
  *
  */
  class Auth
  {
    public function __construct()
    {
      # code...
    }
    public function xx() {
      echo 'xxxxxxxxxxxxxx';
    }
  }

<?php
namespace app\index\controller;
use think\Controller;
use org\Auth;  //引入扩展类
class Index extends Controller
{
  public function index()
  {
    $a = new Auth();
    $a->xx();
  }
}