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