php 获取上个月的起止时间戳

$thismonth = date('m');
$thisyear = date('Y');
if ($thismonth == 1) {
    $lastmonth = 12;
    $lastyear = $thisyear - 1;
} else {
    $lastmonth = $thismonth - 1;
    $lastyear = $thisyear;
}
$lastStartDay = $lastyear . '-' . $lastmonth . '-1';
$lastEndDay = $lastyear . '-' . $lastmonth . '-' . date('t', strtotime($lastStartDay));

$b_time = strtotime($lastStartDay);//上个月的月初时间戳
$e_time = strtotime($lastEndDay);//上个月的月末时间戳