AdamWathan\Form\Elements\Select::renderOptGroup PHP Method

    protected function renderOptGroup($label, $options)
    {
        list($values, $labels) = $this->splitKeysAndValues($options);
        $options = array_map(function ($value, $label) {
            return $this->renderOption($value, $label);
        }, $values, $labels);
        return implode([sprintf('<optgroup label="%s">', $label), implode($options), '</optgroup>']);
    }