This seems to be special case for months with 31 days:
Note that '-1 month' may produce unexpected result when used in last day of month that has 31 days (from http://www.php.net/manual/de/datetime.formats.relative.php#102947)
What you can do is:

$date = new DateTime('last day of last month'); // this is "2012-04-30" now
/// 'first day of last month' would work either, of course

And then it depends on what you are going to do with the date.

我认为您需要有一个已经存在的日期时间并修改它，如下所示：

<?php
$d = new DateTime( date("Y-m-d") );
$d->modify( 'last day of previous month' );
echo $d->format( 'Y-m-d' ), "\n";
?>

这是一个代码块，我希望它能有所帮助：
今天注意的是：2022-12-05

const MONTH_YEAR_DAY_FORMAT = 'Y-m-d';

 
    $startDate_m = new \DateTimeImmutable("first day of this month");
    $endDate_m = new \DateTimeImmutable("last day");
    $startDate_m_1 = new \DateTimeImmutable("first day of previous month");
    $endDate_m_1 = $endDate_m->modify("-1 month");

    echo  $startDate_m->format(self::MONTH_YEAR_DAY_FORMAT);
    echo  $endDate_m ->format(self::MONTH_YEAR_DAY_FORMAT);
    echo  $startDate_m_1 ->format(self::MONTH_YEAR_DAY_FORMAT);
    echo  $endDate_m_1 ->format(self::MONTH_YEAR_DAY_FORMAT);

// result
// startDate_m:   2022-12-01 
// endDate_m:     2022-12-05 
// startDate_m_1: 2022-11-01 
// endDate_m_1:   2022-12-05