function getSubCategories($elements, $parentId = 0) {

    $branch = [];
    foreach($elements as $element) {
        if ($element->parent_id == $parentId) {

            $branch[] = $element;
            
            $children = getSubCategories($elements, $element->id);
            
            if($children)
                $branch = array_merge($branch, $children);
            
        }
    }
    return $branch;

}