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Introduction to test doubles in PHP

Published 26 August, 2021

Test doubles are one of the essential pillars of unit testing.

In this post and the next upcoming ones, I will guide you through all the five test doubles and explain each one in detail:

  • Dummies.
  • Stubs.
  • Mocks.
  • Spies.
  • Fakes.

But before I dig into that, let me show you why do we need test doubles.

Please bear in mind that you need to have PHPUnit up and running on your machine.

What is test double?

According to Martin Fowler the test double is:

A generic term for any case where you replace a production object for testing purposes.

For example, let's say that your application uses an external API to do some currency conversion:

namespace App;

use Exception;
use GuzzleHttp\Client;

class CurrencyConversion
{
    public function __construct(private Client $client)
    {
    }

    public function convert(string $from, string $to, int|float $amount): float
    {
        $query = http_build_query([
            'from' => $from,
            'to' => $to,
            'amount' => $amount,
        ]);

        $req = $this
            ->client
            ->request('GET', 'https://api.exchangerate.host/convert?'.$query);

        if ($req->getStatusCode() !== 200) {
            throw new Exception('Could not convert!');
        }

        $res = json_decode($req->getBody()->getContents());

        if (json_last_error() !== JSON_ERROR_NONE) {
            throw new Exception('Invalid response!');
        }

        return number_format($res->result, 2);
    }
}

The obvious question that comes to mind is how do we test the CurrencyConversion class?

Would it be ok to write a unit test that interacts with an external API?

Let's see:

use App\CurrencyConversion;
use GuzzleHttp\Client;
use PHPUnit\Framework\TestCase;

class CurrencyConversionTest extends TestCase
{
    /** @test */
    public function it_converts_currency()
    {
        $cn = new CurrencyConversion(new Client());

        $amount = $cn->convert('USD', 'DKK', 100);

        $this->assertSame(634.461551, $amount);
    }
}

Run the test:

phpunit --filter="it_converts_currency"

Result:

Time: 00:00.086, Memory: 20.00 MB

OK (1 test, 1 assertion)

The test passes, but there are a few caveats:

  • The test is wrong, yes, you read it correctly, it is wrong because the exchange rate is subject to change, and that happens constantly.
  • We should never call external APIs within our unit tests.
  • Unit test should be as fast as possible, but calling an external API slows down the testing process.
  • We should only test our code - code that’s written by ourselves - but the external API is not ours.

The second point requires a bit of clarification.

You must know that production data - such as API tokens - should not be exposed to the testing environment because the testing environment - as its name implies - tests the production’s behavior without affecting it by making some unwanted changes.

Furthermore, calling production APIs could be dangerous. For example, imagine an API that chargers money - such as Stripe - or an OCR API - such as Google Vision API - where you pay per page/image.

So, what would we do? 😕

Well, we basically, mock the production behaviour. 🚀

Mocking means that we replace a production object (GuzzleHttp\Client) with a testing object; that's it.

In our example, we need to know what response does the API return, then we mock it and send it to the CurrencyConversion.

Sounds confusing? 🤔

Let’s have a look at the API response:

{
  "motd": {
    "msg": "If you or your company use this project or like what we doing, please consider backing us so we can continue maintaining and evolving this project.",
    "url": "https://exchangerate.host/#/donate"
  },
  "success": true,
  "query": {
    "from": "USD",
    "to": "DKK",
    "amount": 100
  },
  "info": {
    "rate": 6.343743
  },
  "historical": false,
  "date": "2021-08-23",
  "result": 634.374255
}

As you have noticed, the CurrencyConvertor::convert method relies on the result key - which is the total converted amount - so all we need to do is sending a fake response and then assert the predefined value.

But why would we assert something that already know? I mean the result value is 634.374255.

That's a good question.

If you look at the CurrencyConvertor::convert method, you'll see that it uses the number_format function, so the goal is to test the behavior of number_format.

How do we write test doubles?

There are a few PHP libraries to deal with test doubles in PHP, or what so-called mocking frameworks:

PHPUnit doesn’t support spies or fakes, but the other frameworks do.

Let’s start testing the CurrencyConvertor using PHPUnit.


use App\CurrencyConversion;
use GuzzleHttp\Client;
use PHPUnit\Framework\TestCase;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\StreamInterface;

class CurrencyConversionTest extends TestCase
{
    public function test_it_converts_currency()
    {
        $client = $this->createMock(Client::class);

        $response = $this->createMock(ResponseInterface::class);

        $response
            ->method('getStatusCode')
            ->willReturn(200);

        $stream = $this->createMock(StreamInterface::class);

        $stream
            ->method('getContents')
            ->willReturn($this->getJson());

        $response
            ->method('getBody')
            ->willReturn($stream);

        $client
            ->method('request')
            ->with('GET', 'https://api.exchangerate.host/convert?from=USD&to=DKK&amount=100')
            ->willReturn($response);

        $currencyConvertor = new CurrencyConversion($client);

        $amount = $currencyConvertor->convert('USD', 'DKK', 100);

        $this->assertSame(634.37, $amount);
    }

    private function getJson(): string
    {
        return '{"motd":{"msg":"If you or your company use this project or like what we doing, please consider backing us so we can continue maintaining and evolving this project.","url":"https://exchangerate.host/#/donate"},"success":true,"query":{"from":"USD","to":"DKK","amount":100},"info":{"rate":6.343743},"historical":false,"date":"2021-08-23","result":634.374255}';
    }
}

Let’s run it:

$ phpunit --filter="it_converts_currency"
PHPUnit 9.5.8 by Sebastian Bergmann and contributors.

.                                                                   1 / 1 (100%)

Time: 00:00.009, Memory: 20.00 MB

OK (1 test, 1 assertion)

The test works as it should be. Now let’s demystify the test case.

Demystify the test case

We’ve started mocking the GuzzleHttp\Client because it is used as a dependency in the CurrencyConvertor::__construct:

$client = $this->createMock(Client::class);

By default, all methods in the GuzzleHttp\Client will be replaced with a dummy implementation that returns null (without calling the original method).

We've got a test double of type Dummy, but we can't really use it, because all the methods return null now.

Back to the CurrencyConvertor::convert method, we see that the request method returns an instance of Psr\Http\Message\ResponseInterface , so we should mock that as well:

$response = $this->createMock(ResponseInterface::class);

$response
    ->method('getStatusCode')
    ->willReturn(200);

The response became a test double of type Stub.

The practice of replacing an object with a test double that (optionally) returns configured return values is referred to as stubbing Stubs in PHPUnit.

So, as you guessed, the difference between Dummies and Stubs is that the latter returns some configured data, whereas the first one will always return null.

In the next post, I will explain Dummies in detail.

The last thing that we need to do is mocking the getBody() method which returns an instance of Psr\Http\Message\StreamInterface:

$stream = $this->createMock(StreamInterface::class);

$stream
    ->method('getContents')
    ->willReturn($this->getJson());

Here we’re telling the mocked object to return json data whenever the getConents() gets called.

And then, we tell the response to return the mocked stream if the getBody() method gets called:

$response
    ->method('getBody')
    ->willReturn($stream);

Lastly, we mock the request method and inject the mocked $client instance as follows:

$client
    ->method('request')
    ->with('GET', 'https://api.exchangerate.host/convert?from=USD&to=DKK&amount=100')
    ->willReturn($response);

$currencyConvertor = new CurrencyConversion($client);

Now, we’re ready to test it:

$amount = $currencyConvertor->convert('USD', 'DKK', 100);

$this->assertSame(634.37, $amount);

So, now you know what mocking is, I will explain all the test doubles in detail in the following posts.

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