[PHP] 해당 주(week)의 날짜 찾기

## find a day of the n'th week in month
## $n = 1'th week, 2nd week, . n'th week
## $w = Sunday(0), Monday(1), ... Saturday(6)
## default) find a day of the first week of sunday in this month
##
function day_of_week_in_month($n=1, $w=0, $month=0, $year=0)
{
  if($n < 1) $n = 1;
  if($w<0 || $w>6) $w = 0;
  if(!$month) $month = date('m');
  if(!$year) $year = date('Y');

  $week = date('w',mktime(1,0,0,$month,1,$year));
  $sunday = ($week==0) ? 1 : (8-$week); // day of first sunday

  $day = ($sunday+$w) + ($n-1)*7;

  return $day;
}

echo day_of_week_in_month(2,1); // 현재달의 두번째주 월(1)요일은 며칠?