Infection + Testo

It's here: Infection 0.33.0 is out with built-in Testo support. Mutation testing now works out of the box.

If you're not yet familiar with mutation testing and why it matters — I have a dedicated article where I unpack why 100% line coverage guarantees nothing, and how mutants find the holes your tests quietly miss.

What "out of the box" means

Until now, Infection knew about three frameworks: PHPUnit, PhpSpec, and Codeception. In 0.33.0 Testo joins the lineup as the fourth.

The Infection Phar archive already contains the adapter inside it — if you run Infection from the Phar, there's nothing extra to install, Testo is picked up automatically.

If you prefer installing Infection via Composer, you'll need a separate package — testo/bridge-infection. Set "testFramework": "testo" in infection.json and register the Codecov plugin with a PhpUnitXmlReportnew PhpUnitXmlReport(string $outputDir)Generates a PHPUnit Coverage XML report — a directory with an index and one file per source. entry in testo.php. After that, you can install the adapter yourself:

composer require --dev infection/infection testo/bridge-infection

Or just run vendor/bin/infection — it will spot testFramework: testo in the config and offer to pull the package for you.

The full walkthrough with every detail lives on the Infection bridge page.

It'll get easier

Right now the configs still need a couple of paths spelled out: a temp directory in infection.json and the coverage-XML location in testo.php. The plan is to hide them inside the adapter so that running Infection doesn't require touching testo.php at all.

Also on the roadmap — dedicated skills for Infection: how to run mutation testing and how to take down surviving mutants more effectively.

If Testo is already in your project and you have a test suite — try running Infection over it. Chances are you're in for a few surprises.