Какие ссылки считаются внешними?

    $timeParts = explode(':', $duration, 3);
    $hours = (int)($timeParts[0] ?? 0);
    $mins = (int)($timeParts[1] ?? 0);
    $secs = (int)($timeParts[2] ?? 0);
    $durationInSeconds = ($hours * 3600) + ($mins * 60) + $secs;

<?php
function sumTimeDurations(array $times): string
{
    $totalSeconds = 0;
    
    foreach ($times as $time) {
        list($hours, $minutes, $seconds) = explode(':', $time);
        $totalSeconds += $hours * 3600 + $minutes * 60 + $seconds;
    }
    
    $hours = floor($totalSeconds / 3600);
    $minutes = floor(($totalSeconds % 3600) / 60);
    $seconds = $totalSeconds % 60;
    
    return sprintf('%02d:%02d:%02d', $hours, $minutes, $seconds);
}

// Ваши примеры времени
$times = [
    '1:05:39',
    '0:55:39',
    '0:15:09',
    '0:36:20'
];

$result = sumTimeDurations($times);
echo $result; // Выведет: 02:52:47
?>

1:05:39 = 3939 секунд
0:55:39 = 3339 секунд
0:15:09 = 909 секунд
0:36:20 = 2180 секунд
-------------------
Сумма: 10367 секунд = 2 часа 52 минуты 47 секунд