php 前一天或后一天的日期

<?php
date_default_timezone_set('PRC'); //默认时区
echo "今天:",date("Y-m-d",time()),"<br>";
echo "今天:",date("Y-m-d",strtotime("18 june 2008")),"<br>";
echo "昨天:",date("Y-m-d",strtotime("-1 day")), "<br>";
echo "明天:",date("Y-m-d",strtotime("+1 day")), "<br>";
echo "一周后:",date("Y-m-d",strtotime("+1 week")), "<br>";
echo "一周零两天四小时两秒后:",date("Y-m-d G:H:s",strtotime("+1 week 2 days 4 hours 2 seconds")), "<br>";
echo "下个星期四:",date("Y-m-d",strtotime("next Thursday")), "<br>";
echo "上个周一:".date("Y-m-d",strtotime("last Monday"))."<br>";
echo "一个月前:".date("Y-m-d",strtotime("last month"))."<br>";
echo "一个月后:".date("Y-m-d",strtotime("+1 month"))."<br>";
echo "十年后:".date("Y-m-d",strtotime("+10 year"))."<br>";
?>