src/modules/Accordion.php
public function renderItems()
    {
        $items = [];
        $index = 0;
        foreach ($this->items as $item) {
            if (!array_key_exists('label', $item)) {
                throw new InvalidConfigException("The 'label' option is required.");
            }
            $items[] = $this->renderItem($item, ++$index);
        }
        return implode("\n", $items);
    }
public function renderItems()
    {
        $items = [];
        $index = 0;
        foreach ($this->items as $item) {
            if (!array_key_exists('label', $item)) {
                throw new InvalidConfigException("The 'label' option is required.");
            }
            $items[] = $this->renderItem($item, ++$index);
        }
        return implode("\n", $items);
    }
Renders collapsible items as specified on [[items]].
@throws InvalidConfigException if label isn't specified
@return string the rendering result