It sounded interesting, so I wrote up something for you.
PHP Code:
<?php
function add_time($t1, $t2) {
// This doesn't have to be explode(), as long as you can explode $t2 into pieces (like substr()'s).
$t2 = explode(':', $t2);
// hh:mm:ss date format. strtotime() will add $t2 components to $t1.
return date('H:i:s', strtotime($t1.' + '.$t2[0].' hours '.$t2[1].' minutes '.$t2[2].' seconds'));
}