As was mentioned in that thread, the way to go about doing it is to split the duration string into it's basic components (hours, minutes, seconds) and sum them individually (handling the overflow properly by carrying over to the next unit of time, e.g. 122 seconds = 2 minutes and 2 seconds).
If you had the DB setup so that there are separate columns for hours, minutes, and seconds then you could do the summing directly from MySQL and have minimal processing to do in the PHP (e.g. the carry-over handling). Otherwise, you'll have to do what that thread mentions - retrieve the rows one-by-one, [man]explode/man them on the separator (e.g. ':'), then sum the individual components.
