thinkphp5 公共函数的使用与调用

<?php
// | Author: xm
// +----------------------------------------------------------------------
use app\index\model\User as UserModel;
// 应用公共文件
function getUserName($user_id) {  //通过user_id来获取 username
    $userName = UserModel::where(‘id‘,$user_id)->value(‘username‘);
    return $userName;
  }