Laravel API Test - How to test API that has external API call

My API code

public function store (Request $request, $profileId)
{
    $all = $request->all();
    $token = AccessToken::with('users')->where('access_token',$request->input('access_token'))->first();

    if($token && $token->users->isOwnProfile($profileId))
    {
        $rules = [
            'access_token'     => 'required',
            'title'            => 'required',
            'description'      => 'required',
            'file_id'          => 'required',
            'audience_control' => 'required|in:' . join(',', PostRepository::$AUDIENCE_CONTROL),
            'tags'             => 'required',
        ];
        $validator = Validator::make($all, $rules);

        $error = $validator->errors()->toArray();
        if ($validator->fails())
        {
            return $this->setStatusCode(401)
                ->setStatusMessage(trans('api.validation failed'))
                ->respondValidationMessage($error);
        }
        try {
            $response = $this->postRepository->save($request, $profileId);
            if(isset($response['error']))
                return $this->messageSet([
                    'message' => $response['error']['message'],
                ], $response['error']['status_code']);

            return $this->setDataType('post_id')
                ->setStatusCode('200')
                ->respondWithCreatedId(trans('api.Post created'), $response->id);
        } catch (\Exception $e) {
            return $this->respondInternalError(trans('api.processing error'));
        }
    }
    return $this->respondInternalError('404 page');

}

From save method it calls another method that calls an external API.

/*
 * this function returns some response where it has profile_id for 
 * the file which in other save function is matched that the
 * profile_id passed as parameter is same with file profile_id
 */
public function getFileDetails($file_id)
{
    try
    {
        $response = json_decode((new Client())->request('GET', env('xyz','http://abc.xyz/api/v1').'/files/' . $file_id)->getBody()->getContents(), true);
    }
    catch (RequestException $e)
    {
        $response = json_decode($e->getResponse()->getBody()->getContents(), true);
    }

    return $response;

}

Now this is my test function for API.

public function testPostCreateChecksProfileMatchesCorrectly()
{
    $this->json('POST', 'profiles/' . $this->getProfile()->id . '/posts' . '?access_token=' . $this->getAccessToken(), [
        'title' => 'api testing title',
        'description' => 'api testing description',
        'audience_control' => 'public',
        'tags' => [
            'Animals',
            'City'
        ],
        'file_id' => '281'
    ])->seeJsonStructure([
        'success' => [
            'message',
            'post_id',
            'status',
        ],
    ]);
}

Now my question is how can I create a fake response for the external API when I am testing.

I am using PHPUnit & Laravel 5.2.