strtotime( )就能解决问题.使用 mktime()将您以前的日期转换为Unix时间戳，然后使用像这样:

strtotime() will do the trick. Convert your previous date to a Unix timestamp using mktime(), then use it like this:

$from_unix_time = mktime(0, 0, 0, $month, $day, $year);
$day_before = strtotime("yesterday", $from_unix_time);
$formatted = date('Y-m-d', $day_before);