how to convert a value to list in laravel code example

Example: laravel return a single dimensional array

$catcount    = 2;
    $parents_ids = [];
    for ($i = 0; $i < $catcount; $i++) {
        $parents_ids[] = $userCategory[$i];
    }
    $subCats = Category::whereIn('parent_id', $parents_ids)->pluck('id');