date_default_timezone_set('PRC');
function get_month_last_day($date = '')
{
if($date != ''){
$time = strtotime($date);
}else{
$time = time();
}
//获取该日期是当月的第几天
$day = date('j',$time);
return date('Y-m-d', strtotime("- {$day} days",$time));
}
echo get_month_last_day('2018-03-25');