private function _extract_values($array)
{
$result = array();
// iterate through the array's values
foreach ($array as $index => $value) {
// if entry is an array, flatten array recursively
if (is_array($value)) {
$result = array_merge($result, $this->_extract_values($value));
} else {
$result[] = $index;
}
}
// return found values
return $result;
}