Format number of seconds to HH:MM:SS.

snippet.php

function format_duration($seconds) {
  $time = round($seconds);

  // Check if video length is longer than 1 hour. If not, we will skip the hour
  // numbers.
  if ($seconds > 60 * 60) {
    $formatted = sprintf('%02d:%02d:%02d', ($time / 3600), ($time / 60 % 60), $time % 60);
  }
  else {
    $formatted = sprintf('%02d:%02d', ($time / 60 % 60), $time % 60);
  }

  return $formatted;
}