wanyue_app/app/model/Serial.php

<?php

namespace app\model;

use support\Db;
use support\Model;

class Serial extends Model
{
    public $table = 'serial';

    protected $primaryKey = 'serial';

    public static function getSerial()
    {
        Db::insert('insert into `app_serial` value(default)');
    }
}