php 时间如何增加1天

<?php
echo "今天:",date(&#39;Y-m-d H:i:s&#39;),"<br>";
echo "明天:",date(&#39;Y-m-d H:i:s&#39;,strtotime(&#39;+1 day&#39;));
?>

echo "明天:",date('Y-m-d H:i:s',strtotime('+1 day'));
echo "明天:",date('Y-m-d H:i:s',strtotime('+1 hour'));
echo "明天:",date('Y-m-d H:i:s',strtotime('+1 minute'));
echo "明天:",date('Y-m-d H:i:s',strtotime('+1 mouth'));

date('Y-m-d H:i:s',strtotime("+1 day +1 hour +1 minute");

echo   date("Y-m-d",strtotime("+1 month",strtotime("2012-02-04")));

echo   date("Y-m-d",strtotime("+1 week",strtotime("2011-02-04")));

echo   date("Y-m-d",strtotime("+1 day",strtotime("2011-02-04")));