- explode: documentation ( source)
<?php
$interval = '50:25:10';
$datestring = '2013-01-05 10:55:15';
list($hours, $minutes, $seconds) = explode(':', $interval);
$date = new DateTime($datestring);
$date->add(new DateInterval('PT'.$hours.'H'.$minutes.'M'.$seconds.'S'));
echo $date->format('Y-m-d H:i:s');