PHP 获取前一天或后一天的日期代码

PHP 判断前/后一天，或前后多少天的代码：

<?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>"; 
?>

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; 
?>