改进后
function get_period_timestamp($start_time, $period_type = 'daily')
{

    switch ($period_type) {

        case "daily":
            //所选时间的昨天起始
            $before_begin = date("Y-m-d 00:00:00", strtotime("-1 day", $start_time));
            $before_end = date("y-m-d 23:59:59", strtotime($before_begin));
            //所选时间的一天起始
            $begin = strtotime(date("Y-m-d", $start_time));
            $end = strtotime(date("Y-m-d 23:59:59", $start_time));
            break;

        case "weekly":

            //上周一
            if (date("w") == 1) {
                $before_begin = date("Y-m-d", strtotime("last monday", $start_time)); //无论今天几号,-1 monday为上一个有效周
            } else {
                $before_begin = date("Y-m-d", strtotime("-1 week last monday", $start_time)); //无论今天几号,-1 monday为上一个有效周
            }

            //上周日
            $before_end = date("Y-m-d  23:59:59", strtotime("-1 sunday", $start_time)); //上一个有效周日,同样适用于其它星期
            //本周一

// $begin = date("Y-m-d", ($start_time - ((date("w") == 0 ? 7 : date("w")) - 1) 24 3600)); //w为星期几的数字形式,这里0为周日
$begin = date("Y-m-d", strtotime("this week Monday", $start_time));
$end = date("Y-m-d", strtotime('+1week', $begin)) - 1;
break;

        case "month":
            //上月一日
            $before_begin = date("Y-m-d",strtotime("first day of -1 month",$start_time));
            //上月最后一日
            $before_end = date("Y-m-d  23:59:59",strtotime("last day of -1 month",$start_time));

            //所选时间的月起始
            $begin = date("Y-m-d", strtotime(date("Y-m", $start_time) . "-01 00:00:00")); //直接以strtotime生成
            $begin = date("Y-m-d", strtotime(date("Y-m-01", $start_time)));
            $end = date("Y-m-d", strtotime('+1month', $begin)) - 1;
            break;

        case "year":

            //去年一月一日
            $before_begin = date("Y-m-d", strtotime("-1 year", strtotime(date("Y", $start_time) . '-01-01 00:00:00'))); //本月一日直接strtotime上减一个月
            //去年十二月三十一日
            $before_end = date("Y-m-d  23:59:59", strtotime(date("Y", $start_time) ."-01-01 00:00:00") - 1); //本月一日减一天即是上月最后一日

            //所选时间的年起始

            $begin = date("Y-m-d", strtotime(date("Y-01-01", $start_time)));
            $end = date("Y-m-d", strtotime('+1year', $begin)) - 1;

            break;
        default:
            return build_result('类型错误 ：daily,weekly,month,year');
    }

    $result = [
        "before_begin" => $before_begin,
        "before_end" => $before_end,
        "begin" => $begin,
        "end" => $end
    ];
    return build_result('查询成功', 0, $result);
}