如：當前時間為：2017-02-28，則獲取：2017-01-01 -> 2017-02-01

$month = date('m');
$year = date('Y');
$last_month = date('m') - 1;
if($month == 1){
 $last_month = 12;
$year = $year - 1;
}
$start_time = mktime(0, 0, 0, $last_month, 1, $year);
$end_time = mktime(0, 0, 0, $month, 1, $year);

或者： 2016-12-31 -> 2017-01-31 （mktime的方法引數調一下就可以了）

$start_time = mktime(0, 0, 0, $last_month, 0, $year);
$end_time = mktime(0, 0, 0, $month, 0, $year);