Most of the array_flatten functions don't allow preservation of keys. Mine allows preserve, don't preserve, and preserve only strings (default).

<?
// recursively reduces deep arrays to single-dimensional arrays
// $preserve_keys: (0=>never, 1=>strings, 2=>always)
function array_flatten($array, $preserve_keys = 1, &$newArray = Array()) {
  foreach ($array as $key => $child) {
    if (is_array($child)) {
      $newArray =& array_flatten($child, $preserve_keys, $newArray);
    } elseif ($preserve_keys + is_string($key) > 1) {
      $newArray[$key] = $child;
    } else {
      $newArray[] = $child;
    }
  }
  return $newArray;
}

// Tests

$array = Array(
  'A' => Array(
    1 => 'foo',
    2 => Array(
      'a' => 'bar'
    )
  ),
  'B' => 'baz'
);

echo 'var_dump($array);'."\n";
var_dump($array);
echo 'var_dump(array_flatten($array, 0));'."\n";
var_dump(array_flatten($array, 0));
echo 'var_dump(array_flatten($array, 1));'."\n";
var_dump(array_flatten($array, 1));
echo 'var_dump(array_flatten($array, 2));'."\n";
var_dump(array_flatten($array, 2));
?>