strtotime 方法返回结果跟预期不符

异常 demo

$yesterday = '2023-10-31';
$last_m = date('Y-m', strtotime('last month',strtotime($yesterday)));
// 期望 $last_m = '2023-09' 实际 $last_m = '2023-10'
$this_m = date('Y-m', strtotime($yesterday));

正常 demo

$yesterday = '2023-10-31';
$this_m = date('Y-m', strtotime($yesterday));
$last_m = date('Y-m', strtotime('last month', strtotime($this_m)));

疑问

是环境配置问题？还是 strtotime 本身的问题？