php怎么获取几月后的日期

<?php
header("Content-type:text/html;charset=utf-8");
echo "当前时间戳为：".strtotime("now")."<br>";
echo "格式化当前时间戳：".date("Y-m-d H:i:s",strtotime("now"))."<br><br>";
echo "1个月后的时间戳：".strtotime("+1 month")."<br>";
echo "格式化时间戳：".date("Y-m-d H:i:s",strtotime("+1 month"))."<br><br>";
echo "当前时间戳后2个月：".date("Y-m-d H:i:s",strtotime("+2 month"))."<br>";
echo "当前时间戳后3天：".date("Y-m-d H:i:s",strtotime("+3 month"))."<br>";
echo "当前时间戳后4天：".date("Y-m-d H:i:s",strtotime("+4 month"))."<br>";
?>

<?php
echo strtotime("now"), "<br />";
echo strtotime("10 September 2000"), "<br />";
echo strtotime("+1 day"), "<br />";
echo strtotime("+1 week"), "<br />";
echo strtotime("+1 week 2 days 4 hours 2 seconds"), "<br />";
echo strtotime("next Thursday"), "<br />";
echo strtotime("last Monday"), "<br />";
?>

<?php
header("Content-type:text/html;charset=utf-8");
$start = &#39;last Monday&#39;;
$interval = strtotime("$start + 4 days");
echo "现在\$interval 表示上周的" . date(&#39;l&#39;,$interval);
?>