PHP Deep merge function with filter

merge.php

<?php
// based on drupal_array_merge_deep_array

function MergeDeep($arrays, $filterFn = null) {
    return _merge_deep_array($arrays, "", 1, $filterFn);
}

function _merge_deep_array($arrays, $path, $depth, $filterFn) {
  // detect if associative or sequential array
  if (array_values($arrays) !== $arrays) {
      return _merge_deep_array(array($arrays), $path, $depth, $filterFn);
  }

  $result = array();
  foreach ($arrays as $array) {
    foreach ($array as $key => $value) {
      // Renumber integer keys as array_merge_recursive() does. Note that PHP
      // automatically converts array keys that are integer strings (e.g., '1')
      // to integers.

      $subpath = (strlen($path) > 0?$path . "." . $key:$key);

      if (!is_null($filterFn) && !$filterFn($subpath, $value, $depth)) {
              continue;
      }

      // TODO: this doesn't work always, imagine 4 => value, 6 => value. It will just count 1 => value, 2 => value.
      if (is_integer($key)) {
        $result[] = $value;
      }
      // Recurse when both values are arrays.
      elseif (isset($result[$key]) && is_array($result[$key]) && is_array($value)) {
        $result[$key] = _merge_deep_array(array($result[$key], $value), $subpath, $depth + 1, $filterFn);
      } elseif (is_array($value)) {
        $result[$key] = _merge_deep_array(array($value), $subpath, $depth + 1, $filterFn);
      }
      // Otherwise, use the latter value, overriding any previous value.
      else {
        $result[$key] = $value;
      }
    }
  }

  return $result;
}

class MergeDeepTest extends PHPUnit_Framework_TestCase
{
    var $src = array(
        'a' => 'b',
        'b' => array(
            'c' => 9,
            'd' => null,
            'e' => array(
                'f' => array('g', 'h'),
                'i' => 'j'
            )
        )
    );

    public function testComplexObject()
    {
        $result = MergeDeep($this->src);

        print_r($result);

        // using json_encoder to compare objects
        return json_encode($this->src) === json_encode($result);
    }

    public function testFilterFunction()
    {
        $result = MergeDeep($this->src, function($path, $value, $depth) {
            // return only path a
            return ($path === 'a');
        });

        print_r($result);

        // using json_encoder to compare objects
        return json_encode($this->src) === json_encode($result);
    }

    public function testFilterDeepFunction()
    {
        $result = MergeDeep($this->src, function($path, $value, $depth) {
            // return everything except b.e.f
            return ($path !== 'b.e.f');
        });

        print_r($result);

        // using json_encoder to compare objects
        return json_encode($this->src) === json_encode($result);
    }
}