PHP extension development: How to improve the testability of custom functions through dependency injection?

Use dependency injection (DI) to improve the testability of custom functions in PHP extensions: Create a DI container and register dependencies and custom functions. Use a DI container to get dependencies in a custom function. Simplify testing by replacing dependencies with mock classes in your tests.

PHP extension development: using dependency injection to improve the testability of custom functions

Introduction
In PHP extension development, custom functions can significantly enhance the functionality and flexibility of the extension. However, the testability of these functions can often be a challenge. By introducing dependency injection (DI), we can significantly improve the testability of custom functions, making them easier to unit and integration test.

Dependency Injection
DI is a design pattern that passes an object's dependencies to an object instead of letting the object create those dependencies itself. In our case, this means that we pass dependencies (such as database connections) to the custom function instead of letting the function create the connection itself.

Create an injectable container
First, we need to create a DI container. In this container we will register our custom function and its dependencies. You can use PHP's built-in dependency injection container class or a third-party DI framework.

// 创建 DI 容器
$container = new Container();

// 注册依赖项和自定义函数
$container->set('db', new PDO('mysql:host=localhost;dbname=test', 'root', ''));
$container->set('customFunction', function($db) {
  // 自定义函数使用传递的依赖项
});

Using DI Container
Now we can use DI container in custom function to get dependencies.

// 使用 DI 容器获取依赖项
$fn = $container->get('customFunction');

// 调用自定义函数，依赖项将自动传递
$fn->handle();

Practical case
Let us consider a custom function send_email, which sends an email to the user. This function relies on the mailer class, which provides email sending functionality.

Before using DI
Without DI, the custom function will directly create the mailer class. This can make testing difficult as we need to mock the mailer class or use a real SMTP server.

// 没有 DI 的自定义函数
function send_email($to, $subject, $message) {
  $mailer = new Mailer();
  $mailer->send($to, $subject, $message);
}

After using DI
After using DI, the custom function can obtain the mailer dependencies from the DI container. This allows us to replace mailer with a mock class in our tests, making testing easier.

// 使用 DI 的自定义函数
function send_email($to, $subject, $message, $container) {
  $mailer = $container->get('mailer');
  $mailer->send($to, $subject, $message);
}

// 测试自定义函数
$container = new Container();
$container->set('mailer', new MockMailer()); // 使用模拟类代替 mailer 类

send_email('test@example.com', 'Test Subject', 'Test Message', $container);

Conclusion
By leveraging dependency injection, we can significantly improve the testability of custom functions. It allows us to easily replace dependencies in tests, making testing faster and more reliable.

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