PHP计算一年有多少周，每周开始日期和结束日期

<?phpheader("Content-type:text/html;charset=utf-8");date_default_timezone_set("Asia/Shanghai");$year = (int)$_GET['year'];$week = (int)$_GET['week'];$weeks = date("W", mktime(0, 0, 0, 12, 28, $year));echo $year . '年一共有' . $weeks . '周<br />';if ($week > $weeks || $week <= 0){$week = 1;}if ($week < 10){$week = '0' . $week;}$timestamp['start'] = strtotime($year . 'W' . $week);$timestamp['end'] = strtotime('+1 week -1 day', $timestamp['start']);echo $year . '年第' . $week . '周开始时间戳：' . $timestamp['start'] . '<br />';echo $year . '年第' . $week . '周结束时间戳：' . $timestamp['end'] . '<br />';echo $year . '年第' . $week . '周开始日期：' . date("Y-m-d", $timestamp['start']) . '<br />';echo $year . '年第' . $week . '周结束日期：' . date("Y-m-d", $timestamp['end']);?>