Using Mutation Testing with Infection PHP

Infection mutates PHP code and evaluates whether the test suite can detect behavioral changes.

What It Does

Mutation testing measures whether tests detect deliberate code changes; it requires coverage support. Test invariants and edge cases, not just one happy path. Boundary assertions are more likely to detect mutations.

Requirements and Compatibility

Use a supported PHP runtime and install any third-party tool or extension explicitly shown in the examples. Commands should be run from the project root unless stated otherwise.

Syntax, Inputs and Results

The examples separate configuration, the smallest working operation and a realistic follow-up task. Adapt file paths and dependency versions to your own project.

Example 1: Basic Use

Mutation testing measures whether tests detect deliberate code changes; it requires coverage support.

composer require --dev infection/infection
XDEBUG_MODE=coverage vendor/bin/infection --threads=2

Example 2: Another Case

Test invariants and edge cases, not just one happy path. Boundary assertions are more likely to detect mutations.

<?php
use PHPUnit\Framework\TestCase;
final class DiscountTest extends TestCase {
    public function testDiscountNeverExceedsPrice(): void {
        $discount = static fn (int $price, int $off): int => max(0, $price - $off);
        self::assertSame(0, $discount(30, 40));
        self::assertGreaterThanOrEqual(0, $discount(100, 5));
    }

    public function test2ArticleBoundary(): void { self::assertSame(8, intdiv(17, 2)); }
}

Practical Example

Coverage requires a driver such as Xdebug with coverage mode enabled.

XDEBUG_MODE=coverage vendor/bin/phpunit --coverage-text
XDEBUG_MODE=coverage vendor/bin/phpunit --coverage-html build/coverage
XDEBUG_MODE=coverage vendor/bin/infection --min-msi=60

For production, apply the compatibility and edge-case limits described below.

Important Behavior and Edge Cases

Mutation scores depend on the selected mutators and test coverage.

Comparison with Related Functions and Tools

Subject	Key difference
Unit tests vs integration tests	Unit tests isolate one behavior; integration tests use actual interfaces, databases or framework components.

Common Beginner Mistakes

Mistake	Correct approach
Writing tests without meaningful assertions	Assert an exact result or expected exception, including at least one boundary case.
Skipping the prerequisite of this specific workflow	Install or configure the named tool and confirm the example can actually run.
Ignoring edge conditions	Mutation scores depend on the selected mutators and test coverage.

Key Takeaways

Infection mutates PHP code and evaluates whether the test suite can detect behavioral changes.
Mutation scores depend on the selected mutators and test coverage.
Keep application configuration, source code and validation tests under version control.

Use these examples as a starting point, then validate the result with the actual PHP build, dependencies and inputs used by your application.