PHP 获取两个日期之间相差天数

//获取两个日期之间相差天数
function diffBetweenTwoDays ($day1, $day2)
{
    $second1 = strtotime($day1);
    $second2 = strtotime($day2);

    if ($second1 < $second2) {
        $tmp = $second2;
        $second2 = $second1;
        $second1 = $tmp;
    }
    return ($second1 - $second2) / 86400 - 1;
}