时间规范：
<?php
class Time
{
const SECOND = 1;
const MINUTE_SECOND = 60;
const HOUR_MINUTE = 60;
const HOUR_SECOND = 3600;
const DAY_SECOND = 86400;
const WEEK_SECOND = 604800;
const MONTH_SECOND = 2592000;
const YEAR_SECOND = 31104000;
const DAY_HOUR = 24;
const WEEK_DAY = 7;
const MONTH_DAY = 30;
const YEAR_MONTH = 12;
const YEAR_DAY = 360;
} 建立一个字典，所有关于时间都用常量代替，没有人会不小心算错时间，也不需要临时计算。只需要这样使用 Time::MONTH_SECOND 代表一个月。两个月再加一周 Time::MONTH_SECOND * 2 + Time::WEEK_SECOND。