how to add array or row to array while foreach looping laravel 5.8

I need to get all products for category and sub categories when click on parent category

so i make loop or recursive loop to get all id for category and subcategory to search for its products

public function tree($category, $cats = array())
{
    $items = category_model::select('id')-where('parent_id', $category)-get();
    foreach ($items as $key=$value)
    {
        //$cats = $value;
        $cats = Arr::add($cats, 'id', $value);
        self::tree($value, $cats);
    }
    return $cats;
}

public function allproduct(Request $request)
{
    return self::tree($request-id);
}

I have tried this code but looping with our result

I need to add this all id to make search for products through this array

via Chebli Mohamed