function howManyDays($startDate,$endDate) {

    $date1  = strtotime($startDate." 0:00:00");
    $date2  = strtotime($endDate." 23:59:59");
    $res    =  (int)(($date2-$date1)/86400);        

return $res;
} 

function howManyDays($startDate,$endDate) {

    $date1  = strtotime($startDate." 0:00:00");
    $date2  = strtotime($endDate." 23:59:59");
    $res    =  (int)(($date2-$date1)/86400);        

return $res;
} 

如果你使用的是PHP 5.3 >，这是目前为止最准确的计算绝对差值的方法:

$earlier = new DateTime("2010-07-06");
$later = new DateTime("2010-07-09");

$abs_diff = $later->diff($earlier)->format("%a"); //3

如果你需要一个相对的(带符号的)天数，可以用这个代替:

$earlier = new DateTime("2010-07-06");
$later = new DateTime("2010-07-09");

$pos_diff = $earlier->diff($later)->format("%r%a"); //3
$neg_diff = $later->diff($earlier)->format("%r%a"); //-3

更多关于php的DateInterval格式可以在这里找到:https://www.php.net/manual/en/dateinterval.format.php

$early_start_date = date2sql($_POST['early_leave_date']);


$date = new DateTime($early_start_date);
$date->modify('+1 day');


$date_a = new DateTime($early_start_date . ' ' . $_POST['start_hr'] . ':' . $_POST['start_mm']);
$date_b = new DateTime($date->format('Y-m-d') . ' ' . $_POST['end_hr'] . ':' . $_POST['end_mm']);

$interval = date_diff($date_a, $date_b);


$time = $interval->format('%h:%i');
$parsed = date_parse($time);
$seconds = $parsed['hour'] * 3600 + $parsed['minute'] * 60;
//        display_error($seconds);

$second3 = $employee_information['shift'] * 60 * 60;

if ($second3 < $seconds)
    display_error(_('Leave time can not be greater than shift time.Please try again........'));
    set_focus('start_hr');
    set_focus('end_hr');
    return FALSE;
}

选择的答案不是最正确的答案，因为它将在UTC之外失败。 根据时区(列表)，可能会有时间调整创建“没有”24小时的日子，这将使计算(60*60*24)失败。

这里有一个例子:

date_default_timezone_set('europe/lisbon');
$time1 = strtotime('2016-03-27');
$time2 = strtotime('2016-03-29');
echo floor( ($time2-$time1) /(60*60*24));
 ^-- the output will be **1**

因此，正确的解决方案是使用DateTime

date_default_timezone_set('europe/lisbon');
$date1 = new DateTime("2016-03-27");
$date2 = new DateTime("2016-03-29");

echo $date2->diff($date1)->format("%a");
 ^-- the output will be **2**

如果你想在开始日期和结束日期之间重复所有的日子，我想出了这个:

$startdatum = $_POST['start']; // starting date
$einddatum = $_POST['eind']; // end date

$now = strtotime($startdatum);
$your_date = strtotime($einddatum);
$datediff = $your_date - $now;
$number = floor($datediff/(60*60*24));

for($i=0;$i <= $number; $i++)
{
    echo date('d-m-Y' ,strtotime("+".$i." day"))."<br>";
}