- strtotime: documentation ( source)
- gmdate: documentation ( source)
<?php
$date = new DateTime();
$date = new DateTime('00:08:27');
$date->modify('00:01:32');
echo $date->format('H:i:s');
$summ = strtotime("00:08:27") + strtotime("00:01:32");
echo $summ;
echo "<br>";
echo gmdate("H:i:s", "3036607799");
echo "<br>";
echo gmdate("H:i:s", $summ);