Алексей Яхненко

$arr = array();
$arr[] = array ("name" => "Название 1", "length" => 100, "count" => 15);
$arr[] = array ("name" => "Название 2", "length" => 99,  "count" => 5);
$arr[] = array ("name" => "Название 3", "length" => 99,  "count" => 3);
$arr[] = array ("name" => "Название 4", "length" => 100, "count" => 25);

$temp = array();
foreach ($arr as $item) {
  $temp[$item['length']] = isset($temp[$item['length']]) ?
    $temp[$item['length']] + $item['count'] :
    $item['count'];
}

$result = array();
foreach ($temp as $length => $count) {
  $result[] = array("length" => $length, "count" => $count);
}

print_r($result);