周 月 日 互轉

1、週轉日，全年第n周的起止日

public function dayFromWeek($week,$year=0){

    $year = empty($year)?date('Y'):$year;
    $year_start = mktime(0,0,0,1,1,$year);

    // 判斷第一天是否爲第一週的開始
    if (1 == date('W',$year_start)){
        $start = $year_start;//把第一天做爲第一週的開始
    }else{

        $start = strtotime('next Monday', $year_start);
    }

    // 第幾周的開始時間
    if (0 == $week){
        $weekday['start'] = strtotime('last Monday', $year_start);
    }elseif (1 == $week){

        $weekday['start'] = $start;
    }else{

        $weekday['start'] = strtotime('+'.($week-0).' monday',$start);
    }

    // 第幾周的結束時間
    $weekday['end'] = strtotime('+1 sunday',$weekday['start']);

    $weekday['start'] = date("Y-m-d",$weekday['start']);
    $weekday['end'] = date("Y-m-d",$weekday['end']);
    return $weekday;
}

2、月起止日

public function dayFromMonth($month, $year=0){

    $year = empty($year)?date('Y'):$year;
    $month = strlen($month)==2?$month:'0'.$month;
    $startDay = $year.'-'.$month.'-01';

    $endDay = date('Y-m-d', strtotime(date('Y-m-01', strtotime($startDay)) . ' +1 month -1 day'));

    return ['start'=>$startDay, 'end'=>$endDay];
}

3、月起止周，當月跨過了哪些周

public function weekFromMonth($month, $year=0){

    $day = $this->dayFromMonth($month, $year);

    return ['start'=>date('W',strtotime($day['start'])),'end'=>date('W',strtotime($day['end']))];
}

4、第幾周在當月的起止日

public function dayFromWM($week,$month,$year=0){

    $year = empty($year)?date('Y'):$year;
    $year_start = mktime(0,0,0,1,1,$year);
    $year_end = mktime(0,0,0,12,31,$year);

    $month_start = $year.'-'.$month.'-01';
    $month_end = date('Y-m-d', strtotime(date('Y-m-01', strtotime($month_start)) . ' +1 month -1 day'));

    // 判斷第一天是否爲第一週的開始
    if (intval(date('W',$year_start))===1){
        $start = $year_start;//把第一天做爲第一週的開始
    }else{

        $start = strtotime('next Monday', $year_start);
    }

    // 第幾周的開始時間
    if ($week===0){
        $weekday['start'] = strtotime('last Monday', $year_start);
    }elseif ($week===1){

        $weekday['start'] = $start;
    }else{

        $weekday['start'] = strtotime('+'.($week-0).' monday',$start);
    }

    // 第幾周的結束時間
    $weekday['end'] = strtotime('+1 sunday',$weekday['start']);

    if (date('m',$weekday['end'])!=$month){
         $weekday['end'] = $month_end;
    }else{
        $weekday['end'] = date("Y-m-d",$weekday['end']);
    }
    if (date('m', $weekday['start'])!=$month){
        $weekday['start'] = $month_start;
    }else{
        $weekday['start'] = date("Y-m-d",$weekday['start']);
    }

    return $weekday;
}