To add test coverage for the PHP SendGrid code using PHPUnit, you need to refactor the existing code. Start by setting up a small project with required dependencies, including PHPUnit and Composer. Then, create a new class `SendEmailHandler` that handles sending emails using SendGrid's API. Refactor this class to remove direct object instantiation and handle response errors gracefully. Finally, add tests for different scenarios, such as handling invalid request data and testing the class's functionality with PHPUnit. By following these steps, you can ensure your PHP code is reliable and maintainable.