我有一个月的周数，并且想用 PHP 获取该周内的第一个日期

Question

假设我有以下给定信息：

$year = 2020; 
$month = 7; 

$week_nr_of_month = 2; 

$first_date_in_that_week = ?;

我想知道该月该周内的第一个日期。所以在我目前的情况下输出应该是 2020-07-06

我该如何计算这个？谢谢。

Answer 1

我有这样的事情：<?phpfunction myDate(int $year, int $month, int $week): string {&nbsp; $firstDayDate = DateTime::createFromFormat('j-n-Y', '1-'.$month.'-'.$year);&nbsp; $firstDayDate->setTime(0,0);&nbsp; $firstMondayDate = (clone $firstDayDate)->modify('first monday of this month');&nbsp; $modificator = $week - 2;&nbsp; if ($firstMondayDate->format('d') === $firstDayDate->format('d')) {&nbsp; &nbsp; $modificator++;&nbsp; }&nbsp; if ($modificator < 0) {&nbsp; &nbsp; return $firstDayDate->format('Y-m-d');&nbsp; }&nbsp; $wantedWeekDate = (clone $firstMondayDate)->add(new DateInterval('P'.($modificator*7).'D'));&nbsp;&nbsp;&nbsp; return $wantedWeekDate->format('Y-m-d');}var_dump(myDate(2020, 7, 1),myDate(2020, 7, 2),myDate(2020, 7, 3),myDate(2020, 7, 4),myDate(2020, 7, 5),myDate(2020, 7, 6),myDate(2020, 8, 1),myDate(2020, 8, 2),myDate(2020, 8, 3),myDate(2020, 8, 4),myDate(2020, 8, 5),myDate(2020, 8, 6),myDate(2014, 12, 1),myDate(2014, 12, 2),myDate(2014, 12, 3));测试输出：string(10) "2020-07-01"string(10) "2020-07-06"string(10) "2020-07-13"string(10) "2020-07-20"string(10) "2020-07-27"string(10) "2020-08-03"string(10) "2020-08-01"string(10) "2020-08-03"string(10) "2020-08-10"string(10) "2020-08-17"string(10) "2020-08-24"string(10) "2020-08-31"string(10) "2014-12-01"string(10) "2014-12-08"string(10) "2014-12-15"

Answer 2

这是我的解决方案：function firstDayOfWeekInMonth($year, $month, $week){&nbsp; &nbsp; $firstOfMonth = strtotime("$year-$month-1");&nbsp; &nbsp; $format = 'Y-m-d';&nbsp; &nbsp; if ($week == 1) {&nbsp; &nbsp; &nbsp; &nbsp; return date($format, $firstOfMonth);&nbsp; &nbsp; } else {&nbsp; &nbsp; &nbsp; &nbsp; $secondsInOneDay = 60 * 60 * 24;&nbsp; &nbsp; &nbsp; &nbsp; $secondInOneWeek = $secondsInOneDay * 7;&nbsp; &nbsp; &nbsp; &nbsp; $dayNumberOfFirstOfMonth = date('w', $firstOfMonth); //eg Monday = 1, Tuesday = 2, etc.&nbsp; &nbsp; &nbsp; &nbsp; $offsetToStartOfWeek = ($dayNumberOfFirstOfMonth - 1) * $secondsInOneDay; // eg Monday is 0 days from Monday, Tuesday is 1 day from Monday etc.&nbsp; &nbsp; &nbsp; &nbsp; $startOfWeek1 = $firstOfMonth - $offsetToStartOfWeek; //could be in previous month&nbsp; &nbsp; &nbsp; &nbsp; $StartOfWeekN = $startOfWeek1 + ($week - 1) * $secondsInOneWeek; // week-2 is 1-week ahead of week-1's Monday&nbsp; &nbsp; &nbsp; &nbsp; return date($format, $StartOfWeekN);&nbsp; &nbsp; }}该算法使用自 Unix 纪元以来的秒数。这是一个整数，因此所有计算都是通过整数相加或相乘来完成的，这应该相当快。日期和整数之间的转换仅在函数开始和结束时发生一次。顺便说一句，当我指的是如此多不同的周/月的开始时，我发现为一周的开始找到有意义的名称是一个很大的挑战。为了进行比较，我使用了与布莱希的答案相同的测试：var_dump(firstDayOfWeekInMonth(2020, 7, 1),firstDayOfWeekInMonth(2020, 7, 2),firstDayOfWeekInMonth(2020, 7, 3),firstDayOfWeekInMonth(2020, 7, 4),firstDayOfWeekInMonth(2020, 7, 5),firstDayOfWeekInMonth(2020, 7, 6),firstDayOfWeekInMonth(2020, 8, 1),firstDayOfWeekInMonth(2020, 8, 2),firstDayOfWeekInMonth(2020, 8, 3),firstDayOfWeekInMonth(2020, 8, 4),firstDayOfWeekInMonth(2020, 8, 5),firstDayOfWeekInMonth(2020, 8, 6),firstDayOfWeekInMonth(2014, 12, 1),firstDayOfWeekInMonth(2014, 12, 2),firstDayOfWeekInMonth(2014, 12, 3));string(10) "2020-07-01"string(10) "2020-07-06"string(10) "2020-07-13"string(10) "2020-07-20"string(10) "2020-07-27"string(10) "2020-08-03"string(10) "2020-08-01"string(10) "2020-08-03"string(10) "2020-08-10"string(10) "2020-08-17"string(10) "2020-08-24"string(10) "2020-08-31"string(10) "2014-12-01"string(10) "2014-12-08"string(10) "2014-12-15"