If the you know the times will be less than an hour, you could just use the date()
or $date->format()
functions.
$minsandsecs = date('i:s',$numberofsecs);
This works because the system epoch time begins at midnight (on 1 Jan 1970, but that's not important for you).
If it's an hour or more but less than a day, you could output it in hours:mins:secs format with `
$hoursminsandsecs = date('H:i:s',$numberofsecs);
For more than a day, you'll need to use modulus to calculate the number of days, as this is where the start date of the epoch would become relevant.
Hope that helps.