Somar horas com php

sum.time.php

<?php

function sumHours($firstHour, $secondHour) {
    $baseDate = date('Y-m-d');
    $baseTime = strtotime($baseDate . ' 00:00:00');

    $firstTime = strtotime($baseDate . ' ' . $firstHour) - $baseTime;
    $secondTime = strtotime($baseDate . ' ' . $secondHour) - $baseTime;

    $resultTime = $firstTime + $secondTime;
    return date('H:i:s', $baseTime + $resultTime);
}

echo sumHours('00:01:00', '00:00:50');