FluidTYPO3\Vhs\ViewHelpers\Iterator\ExtractViewHelper::recursivelyExtractKey PHP Method

    public function recursivelyExtractKey($iterator, $key)
    {
        $content = [];
        foreach ($iterator as $v) {
            // Lets see if we find something directly:
            $result = ObjectAccess::getPropertyPath($v, $key);
            if (null !== $result) {
                $content[] = $result;
            } elseif (true === is_array($v) || true === $v instanceof \Traversable) {
                $content[] = $this->recursivelyExtractKey($v, $key);
            }
        }
        $content = $this->flattenArray($content);
        return $content;
    }