Simple transformer for Laravel

SimpleTransformer.php

<?php

namespace App\Domain\Transformer;

use Illuminate\Contracts\Pagination\LengthAwarePaginator as PaginatorContract;
use Illuminate\Pagination\LengthAwarePaginator;
use Illuminate\Pagination\Paginator;
use Illuminate\Support\Collection;
use MongoDB\Model\BSONArray;

//TODO: Write some unit tests for this class
abstract class Transformer
{
    /**
     * Transform a single item.
     *
     * @param  mixed $item
     * @return array
     */
    abstract public function transformItem($item);

    /**
     * Transform the data using one of the transformers available.
     *
     * @param  mixed $data
     * @return mixed
     */
    public function transform($data)
    {
        if ($data instanceof PaginatorContract) {
            return $this->transformPaginator($data);
        }

        if ($data instanceof Collection) {
            return $this->transformCollection($data);
        }

        if ($data instanceof BSONArray) {
            $data = iterator_to_array($data);
        }

        if (is_array($data)) {
            return $this->transformArray($data);
        }

        return $this->transformItem($data);
    }

    /**
     * Transform the paginator data and return a new paginator instance.
     *
     * @param  LengthAwarePaginator $paginator
     * @return LenghtAwarePaginator
     */
    private function transformPaginator(LengthAwarePaginator $paginator)
    {
        $items = array_map([$this, 'transformItem'], $paginator->items());

        return new LengthAwarePaginator(
            $items,
            $paginator->total(),
            $paginator->perPage(),
            $paginator->currentPage(),
            [
                'path' => Paginator::resolveCurrentPath(),
                'pageName' => $paginator->getPageName()
            ]
        );
    }

    /**
     * Transform a collection data and return a new collection.
     *
     * @param  Collection $collection
     * @return Collection
     */
    private function transformCollection(Collection $collection)
    {
        $items = array_map([$this, 'transformItem'], $collection->all());

        return collect($items);
    }

    /**
     * Transforms a simple array
     *
     * @param  array  $data
     * @return array
     */
    private function transformArray(array $data)
    {
        return array_map([$this, 'transformItem'], $data);
    }

    /**
     * Transform the data into a CSV string.
     *
     * @param  mixed $data
     * @return string
     */
    public function toCsv($data)
    {
        $transformed = $this->transform($data);

        if ($data instanceof PaginatorContract) {
            $transformed = $transformed->items();
        }

        if ($data instanceof Collection) {
            $transformed = $transformed->all();
        }

        if (!count($transformed)) {
            return [];
        }

        ob_start();
        $handler = fopen("php://output", 'w');
        fputcsv($handler, array_keys(reset($transformed))); //putting the headers in the csv
        foreach ($transformed as $item) {
            fputcsv($handler, $item);
        }
        fclose($handler);

        return ob_get_clean();
    }
}