Infection

Infection is a mutation testing tool for PHP. Testo connects to it through a dedicated adapter, php-testo/bridge-infection.

Package

php-testo/bridge-infection — Infection extension. Auto-registers via Composer. No plugin needs to be added to testo.php.

Installation

Install Infection and the integration package as dev dependencies:

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

If you use the Phar archive of Infection, you don't need to install the adapter separately — it is already bundled inside the Phar.

Configuration

Infection requires two reports from Testo:

• Coverage XML — a directory of XML files in PHPUnit's coverage format. Tells Infection which tests cover which lines, so it can run only the relevant subset for each mutant. Required.
• JUnit XML — a single test-results file. Helps Infection quickly map a test to the file it lives in, so Testo's filters can be used more efficiently. Optional, but strongly recommended.

Infection looks for both reports in a fixed layout:

└── <tmpDir>/
    └── infection/
        ├── coverage-xml/*.xml
        └── junit.xml

Important:

tmpDir has to be set in Infection's configuration, and the path to coverage-xml — in testo.php.

infection.json

In Infection's configuration (usually infection.json), declare that you're using Testo and set the temporary directory:

{
    "$schema": "vendor/infection/infection/resources/schema.json",
    "source": {
        "directories": ["src"]
    },
    "testFramework": "testo",
    "tmpDir": "runtime",
    "logs": {
        "text": "runtime/infection.log",
        "html": "runtime/infection.html"
    }
}

testo.php

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 pointing at <tmpDir>/infection/coverage-xml:

use Testo\Application\Config\ApplicationConfig;
use Testo\Codecov\CodecovPlugin;
use Testo\Codecov\Report\PhpUnitXmlReport;

return new ApplicationConfig(
    src: ['src'],
    plugins: [
        new CodecovPlugin(
            reports: [
                new PhpUnitXmlReport(__DIR__ . '/runtime/infection/coverage-xml'),
            ],
        ),
    ],
);

Coverage requires the PCOV or XDebug extension. See the Codecov page for setup details and the trade-offs between the two.

Running

XDEBUG_MODE=coverage vendor/bin/infection

PCOV works too — driver requirements come from Codecov, not from Infection.

Infection runs Testo twice:

1. Initial run — executes the full test suite against the original source, collecting coverage and a JUnit log. This is the slow phase: every test runs and the coverage driver is active.
2. Mutant runs — for each mutant, Infection picks only the tests that touch the mutated lines and runs them. Coverage is not collected here, so each mutant runs quickly.

Reusing existing coverage

If a coverage report has already been generated — for example, in an earlier CI step — you can hand it to Infection instead of triggering a fresh Testo run via the --coverage flag:

vendor/bin/infection --coverage=runtime/infection

In this mode Infection skips its own initial run, and the supplied directory must already contain the reports.

To produce them, run Testo with --coverage --log-junit=<tmpDir>/infection/junit.xml — the same flags the adapter uses for the initial run. The folder structure and file names must match what Infection expects.