PHP の日付操作

//返回指定月份的第一天
  function get_the_month_first_day_str($theyear, $themonth) {
    return date("Y-m-d", mktime(0, 0, 0, $themonth, 1, $theyear));
  }
  
  //返回指定月份的最后一天
  function get_the_month_last_day_str($theyear, $themonth) {
    $theday = date("t", mktime(0, 0, 0, $themonth, 1, $theyear));
    return date("Y-m-d", mktime(0, 0, 0, $themonth, $theday, $theyear));
  }
  
  //返回指定日期所在周的第一天(周日为第一天，周一为第二天，以此类推)
  function get_the_week_first_day_str($theyear, $themonth, $theday) {
  $date = mktime(0, 0, 0, $themonth, $theday, $theyear);
  $date = $date - (24 * 60 * 60 * date("w", $date));
  return date("Y-m-d", $date);
  }
  
  //返回指定日期所在周的最后一天(周日为第一天，周一为第二天，以此类推)
  function get_the_week_last_day_str($theyear, $themonth, $theday) {
  $date = mktime(0, 0, 0, $themonth, $theday, $theyear);
  $date = $date - (24 * 60 * 60 * date("w", $date)) + (24 * 60 * 60 * 6);
  return date("Y-m-d", $date);
  }