PHP: Calculate duration between two dates

<?php
$date11 = '2013-03-12';
$date22 = '2013-03-15';
$date11 = strtotime($date11);
$date22 = strtotime($date22);
$diff = $date22 - $date11;
$diff_in_days = floor($diff/(60*60*24));
echo $diff_in_days; // 3
?>

<?php
$date11 = '2013-03-15';
$date22 = '2013-03-12';
$date11 = strtotime($date11);
$date22 = strtotime($date22);
$diff = $date22 - $date11;
$diff_in_days = floor($diff/(60*60*24));
echo $diff_in_days; // -3
?>