php求两个日期之间相差天数的方法

<?php
/**
 * 求两个日期之间相差的天数
 * (针对1970年1月1日之后，求之前可以采用泰勒公式)
 * @param string $day1
 * @param string $day2
 * @return number
 */
function diffBetweenTwoDays ($day1, $day2)
{
  $second1 = strtotime($day1);
  $second2 = strtotime($day2);
   
  if ($second1 < $second2) {
    $tmp = $second2;
    $second2 = $second1;
    $second1 = $tmp;
  }
  return ($second1 - $second2) / 86400;
}
$day1 = "2013-07-27";
$day2 = "2013-08-04";
$diff = diffBetweenTwoDays($day1, $day2);
echo $diff."\n";