- floor: documentation ( source)
- date: documentation ( source)
- strtotime: documentation ( source)
<?php
$spd = (60 * 60 * 24);
$sph = (60 * 60);
$spm = (60);
$start = date('Y-m-d H:m:s');
$end = date('Y-m-d H:m:s', strtotime($start . ' +13 hours'));
$sec_diff = strtotime($end) - strtotime($start);
$days = floor($sec_diff / $spd);
$hours = ($sec_diff - ($days * $spd)) / $sph;
echo $days . " " . $hours;
?>