Как вывести массив в строку?

$al_cat = array_flatten(request(['category']));
        $albums = '['.implode(',',$al_cat).']';
        $album->attachCategories($albums);

public function syncCategories($categories, bool $detaching = true)
    {
        // Find categories
        $categories = $this->prepareCategoryIds($categories);

        // Sync model categories
        $this->categories()->sync($categories, $detaching);

        return $this;
    }

    public function attachCategories($categories)
    {
        return $this->syncCategories($categories, false);
    }

    protected function prepareCategoryIds($categories): array
    {
        // Convert collection to plain array
        if ($categories instanceof BaseCollection && is_string($categories->first())) {
            $categories = $categories->toArray();
        }

        // Find categories by slug, and get their IDs
        if (is_string($categories) || (is_array($categories) && is_string(array_first($categories)))) {
            $categories = app('rinvex.categories.category')->whereIn('slug', $categories)->get()->pluck('id');
        }

        if ($categories instanceof Model) {
            return [$categories->getKey()];
        }

        if ($categories instanceof Collection) {
            return $categories->modelKeys();
        }

        if ($categories instanceof BaseCollection) {
            return $categories->toArray();
        }

        return (array) $categories;
    }