如何查找和替换数组中的间隔？

I have a function that returns arrays of month intervals like this:

array("feb","mar","apr","oct","nov");

and it needs to be changed into this:

array("feb-apr","oct-nov");

How do I achieve this?

Below I have some example code that I've tried, but it throws an 'undefined offset: 13' notice.

$months = array("jan","feb","mar","apr","may","jun","jul","aug","sep","oct","nov","dec");
    $feature_values = array("feb","mar","apr","oct","nov");

    $result = $feature_values[0];
    $start_index = array_search($result, $months);
    $value_count = count($feature_values);

    for ($i = 0; $i < $value_count; $i++) {
      if ($months[$start_index + $i] !== $feature_values[$i]) {
          $result .= ' - ' . $feature_values[$i - 1];
          if ($i < $value_count - 1) {
            $result .= ', ' . $feature_values[$i];
              $start_index = array_search($feature_values[$i], $months);
          }
      }
    }
    return $result;