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>";    
?>

php判断一个日期距今天还有多少天

<?php
echo (strtotime('2020-5-20′)-strtotime(date(“Y-m-d”)))/86400;
?>