Can I (Really) Mock Private Methods in PHPUnit?

How to Mock Private Methods in PHPUnit

Problem:

In certain scenarios, testing specific sections of a class can be challenging due to the presence of private methods that are called internally. How can we stub the results of a private method to test the desired code?

Answer:

Importance of Public API:

Typically, when testing a class, it's recommended to focus on its public API as this represents the contract that your class offers. Private and protected methods are implementation details and should not affect the stability of your tests.

Testing Global State Modifications:

In some cases, a private method may contain logic that depends on global state, making it difficult to test. To address this, consider refactoring your code to eliminate global state and introduce dependencies that can be injected into the class.

Using Mocking Framework for Mocking Private Methods:

If it's unavoidable to test a private method directly, PHPUnit provides a way to mock private methods using its mocking framework. However, this approach is generally discouraged as it breaks encapsulation and may lead to over-reliance on implementation details.

To mock a private method using PHPUnit, you can follow these steps:

1. Create a mock of the class using $testMe = $this->getMock("a", array("c"));
2. Set up the expected behavior using the expects() and method() methods.
3. Execute your tests against the mocked class.

Quote from "Pragmatic Unit Testing":

"In general, you don't want to break any encapsulation for the sake of testing... Most of the time, you should be able to test a class by exercising its public methods."

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