All 7 Unit Testing with PHPUnit questions
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What are assertions in PHPUnit, and how do commonly used assertions like assertEquals and assertTrue let you verify expected behavior within a test? What is the arrange act assert pattern, and how does structuring a test around these three distinct steps make tests clearer and easier to understand? What are mock objects in PHPUnit, and why are they useful when testing a piece of code that depends on an external service, such as sending an email or calling a third party API? What is test coverage, and why should achieving a high test coverage percentage not necessarily be treated as the sole ultimate goal of a testing strategy? How do data providers in PHPUnit let you run the exact same test logic repeatedly against many different sets of input data, without duplicating the test method itself? How does test driven development, commonly abbreviated as TDD, change the typical order in which tests and actual implementation code are written, and what benefits does this workflow claim to offer?
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What is PHPUnit, and what is a unit test meant to actually verify about a piece of code?

Beginner

Answer

PHPUnit is the most widely used testing framework in the PHP ecosystem, providing the tools needed to write, organize, and run automated tests against your code, and a unit test specifically focuses on verifying that one small, isolated piece of code, typically a single function or method, behaves exactly as expected when given a specific set of inputs, checking that it produces the correct output or correctly performs the expected action, without needing to manually and repeatedly test that same logic by hand every single time a change is made.
public function testAddReturnsCorrectSum(): void
{
    $calculator = new Calculator();
    $this->assertEquals(5, $calculator->add(2, 3));
}
Real-world example A developer writes a unit test verifying that a Calculator class's add method correctly returns five when given the inputs two and three, and can now run that same test automatically at any time to instantly confirm that behavior still works correctly.

Common follow-ups: What is the difference between a unit test and an integration test?;How do you actually run PHPUnit tests from the command line?

Static Analysis (PHPStan & Psalm);Functions & Scope
All 7 Unit Testing with PHPUnit questions