Infection

Infection is a command-line mutation-testing library for PHP, aimed at teams that want to see whether their tests catch plausible code changes. It is a strong fit for PHPUnit and other supported PHP test suites that need mutation scores in CI, but parallel runs need care when tests share state.

Overview

Rather than merely checking which lines tests execute, Infection changes PHP code through predefined mutation operators and runs tests against those changes. It records whether each mutant is killed, survives, errors, or times out, giving teams a sharper measure of whether covered code is actually protected by assertions.

Its Mutation Score Indicator (MSI) reports the percentage of generated mutations detected by the test suite. Mutation Code Coverage and Covered Code Mutation Score Indicator add coverage-aware views, helping teams distinguish a weak test assertion from a mutant that tests never reached.

Key features

Scores and CI gates

The --min-msi and --min-covered-msi options can fail a build when scores fall below configured thresholds. That makes Infection useful for enforcing test-quality expectations, though teams should choose thresholds with their codebase and suite in mind rather than treating a single score as a complete measure of quality. It can also emit GitHub annotations and GitLab Code Quality reports.

Targeted analysis and parallel runs

With --git-diff-lines, Infection can limit mutation work to touched lines; --git-diff-filter can narrow it to changed files. These options make incremental analysis more practical during active development than rerunning mutation testing across an entire project for every change. The --threads option runs tests for mutants in parallel, with --threads=max detecting CPU cores automatically. But parallel execution can cause false positives when tests depend on one another or use a database, so that speed-up is not safe for every suite.

Mutators and integrations

The project describes more than 100 mutators, including arithmetic, boolean, equality, conditional-boundary, return-value and visibility changes, organized into profiles. Custom mutator support extends the set of changes teams can examine. Optional PHPStan and Mago integrations can identify escaped mutants involving type violations, dead code and unreachable paths.

Infection supports PHPUnit, PhpSpec, Codeception and Testo. It can publish mutation badges and HTML reports through Stryker Dashboard using a project API key. The browser-based Infection Playground lets users try PHP code and tests without installing Composer, Infection or PHPUnit, a useful way to explore the method before adding it to a project.

Installation and security

Installation options include PHAR, Phive, Composer, Git and Homebrew. The recommended PHAR bundles the four supported test frameworks, and its signature can be verified using the documented GPG key. Infection is released under the BSD-3-Clause license. Its security policy supports only the latest version, though older releases may be patched depending on vulnerability severity; security issues should be reported privately on GitHub. Community help is available through Discord and GitHub Discussions, and the project welcomes issues and pull requests.

Pricing

Infection is free, with a free plan and no paid tiers to weigh against its core mutation-testing capabilities. Its BSD-3-Clause license is suitable for use in open-source and commercial projects. The main costs are operational: teams need a compatible PHP runtime and test runner, and mutation testing adds work beyond an ordinary test run.

Platforms

Infection supports Linux and macOS, and offers a web playground. It targets PHP rather than multiple programming languages. Current documentation requires PHP 8.3 or newer plus Xdebug, phpdbg or pcov, so older PHP environments cannot meet its documented runtime requirements.

Who it's for

Infection suits PHP teams that already maintain automated tests and want to measure whether those tests detect code-level faults, particularly when mutation thresholds and changed-code analysis can fit into CI. Its range of test-framework integrations and reporting options makes it relevant beyond PHPUnit-only projects.

It is less suitable for teams on older PHP runtimes, projects written in other languages, or suites whose shared state makes parallel mutation runs unreliable. Teams can still use targeted analysis, but should not assume that more threads always mean trustworthy results.

Pros and cons

• Pro: Multiple mutation and coverage-aware metrics, plus score thresholds, make test effectiveness measurable and enforceable in CI.
• Pro: Changed-line and changed-file modes focus analysis on current work, avoiding a full-project mutation run for every edit.
• Pro: Four supported PHP test frameworks, several installation routes and the browser playground lower adoption friction for different workflows.
• Con: PHP 8.3 or newer and a supported coverage driver are required, excluding environments that cannot meet those prerequisites.
• Con: Parallel runs can report false positives for interdependent or database-using tests, limiting the reliability of automatic speed-ups.
• Con: The security policy centers support on the latest version, so teams that stay on older releases may have less assurance of a patch.

Alternatives

For open-source projects wanting a freemium alternative with a free tier and trial, consider ArcMutate; its Base plan is $8 a month. Mutant is another freemium option: public repositories get its full feature set free, while its Commercial monthly plan is $30 a month per developer for any number of repositories. For a free, open-source PHP-oriented choice, Mutatest is an alternative.

PIT is a free mutation-testing alternative. muttest is another free option. Teams working in JavaScript, TypeScript, C# or Scala should consider Stryker Mutator, a free open-source tool for those languages. Cosmic Ray and Gambit are also free alternatives.

For more options, browse Mutation Testing Tools.

Verdict

PHP teams seeking a free, CI-ready way to check whether tests catch realistic code changes should choose Infection, especially if changed-code analysis can keep runs focused. Its broad mutator set, score gates and framework support make it a substantial testing tool rather than a simple coverage counter. Look elsewhere if the project is not PHP, the runtime cannot reach PHP 8.3, or shared-state tests make parallel execution too risky.