protected function _getRealOffset($offset)
    {
        /* Optimize: check for base $offset in array first. */
        if (parent::offsetExists($offset)) {
            return $offset;
        }

        foreach (array_keys($this->getArrayCopy()) as $key) {
            if (strcasecmp($key, $offset) === 0) {
                return $key;
            }
        }

        return null;
    }