<?php

  $array = Array (
    4894 => Array (
      0 => Array (
        'Informative' => Array (
          'id' => 109
        ),
        'jQuery' => Array (
          'length' => '00:04:03',
          'alignment' => 8
        )
      ),
      1 => Array (
        'Informative' => Array (
          'id' => 110
        ),
        'jQuery' => Array (
          'length' => '00:06:03',
          'alignment' => 8
        )
      )
    )
  );

  $totalTimeSecs = 0;
  foreach ($array as $l1) { // Loop outer array
    foreach ($l1 as $l2) { // Loop inner arrays
      if (isset($l2['jQuery']['length'])) { // Check this item has a length
        list($hours,$mins,$secs) = explode(':',$l2['jQuery']['length']); // Split into H:m:s
        $totalTimeSecs += (int) ltrim($secs,'0'); // Add seconds to total
        $totalTimeSecs += ((int) ltrim($mins,'0')) * 60; // Add minutes to total
        $totalTimeSecs += ((int) ltrim($hours,'0')) * 3600; // Add hours to total
      }
    }
  }
  echo "Total seconds: $totalTimeSecs\n";

  $hours = str_pad(floor($totalTimeSecs / 3600),2,'0',STR_PAD_LEFT);
  $mins = str_pad(floor(($totalTimeSecs % 3600) / 60),2,'0',STR_PAD_LEFT);
  $secs = str_pad($totalTimeSecs % 60,2,'0',STR_PAD_LEFT);
  echo "Total time string: $hours:$mins:$secs";

Total seconds: 606
Total time string: 00:10:06