When Should You Mock Private Methods in PHPUnit?

Mocking Private Methods with PHPUnit

Introduction

Mocking private methods can be a useful technique for testing the logic within a class without exposing its implementation details. However, it's important to approach this with caution to avoid breaking encapsulation and compromising the integrity of your code.

Problem

Consider the following example:

<code class="php">class A {
  public function b() { 
    // some code
    $this->c(); 
    // some more code
  }

  private function c(){ 
    // some code
  }
}</code>

How can you stub the result of the private method c() to test the "some more code" part of the public function b() using PHPUnit?

Solution

Option 1: Consider Refactoring

Generally, it's not considered best practice to test private methods directly. Instead, focus on testing the public API of the class. Any internal implementation details should be considered a black box.

Option 2: Utilize Mocks (Proceed with Caution)

However, in certain scenarios, mocking private methods may be necessary. To achieve this using PHPUnit:

1. Create a mock object for the class under test:

   <code class="php">$mock = $this->getMockBuilder('A')
               ->disableOriginalConstructor()
               ->getMock();</code>

2. Define the expected behavior of the private method:

   <code class="php">$mock->expects($this->once())
       ->method('c')
       ->will($this->returnValue(YOUR_STUBBED_VALUE));</code>

3. Replace the original instance with the mock in your test:

   <code class="php">$originalInstance = new A();
   $this->reflection()->setProtectedProperty(
      $originalInstance,
      'c',
      $mock
   );</code>

4. Run your test as usual.

Conclusion

While it may be tempting to mock private methods for testing purposes, it's important to prioritize the principles of encapsulation and avoid exposing internal details unnecessarily. Consider refactoring your code to enable testing without relying on private method mocking.

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