Calculating time differences.

If I have two times, in text format: 14:32:23 and 17:56:36 How can I calculate the difference between the two, and have it returned like: 03:24:13 Then beyond that, once I have a bunch of these differences, such as: 03:24:13 00:45:52 etc I need be able to add those all up as I go, so it'd be a loop and it'd keep a running total. The numerical equivalent of what I want to do is: $total = 0; while ($data = mysql_fetch_array($result)) { $difference = $data['end'] - $data['start']; // for example, (end = 5) - (start = 3) echo $difference; // 2 $total += $difference; } echo $total; Mike