gistfile1.php

<?php

require 'vendor/autoload.php';

class Transformer extends League\Fractal\TransformerAbstract
{
    protected $availableIncludes = ['child'];
    protected $defaultIncludes = ['child'];

    public function transform($data)
    {
        return $data;
    }

    public function includeChild($data)
    {
        return $this->item($data['child'], function ($child) {
            return $child;
        });
    }
}

$item = [
    'foo' => 'bar',
    'child' => []
];

$manager = (new League\Fractal\Manager)->setSerializer(new League\Fractal\Serializer\ArraySerializer);
$resource = new League\Fractal\Resource\Item($item, new Transformer);
$scope = $manager->createData($resource);
echo $scope->toJson();

/**
Output:
{"foo":"bar","child":[]}

Desired:
{"foo":"bar","child":{}}
*/