１５分刻みにしたときの次回の時間を計算。応用して分刻みの時刻表も可能

public function calc_datetime ($margin_minutes=15) {
    $timestamp = strtotime(date('Y-m-d H:i:s'));
//    $timestamp = strtotime(date('Y-m-d H:i:s', strtotime('2023-01-01, 00:00:00')));  時間指定する場合
    $year = date('Y', $timestamp);
    $month = date('m', $timestamp);
    $day = date('d', $timestamp);
    $hour = date('H', $timestamp);
    $minute = date('i', $timestamp);
    if($minute % $margin_minutes) {
      $minute += $margin_minutes - ($minute % $margin_minutes);
    } else {
      $minute += $margin_minutes;
    }
    $datetime = date('Y-m-d H:i:s', mktime($hour, $minute, 0, $month, $day, $year));
    return $datetime;
  }