Code coverage alone can give false confidence — a test suite with 100% coverage can still miss critical regressions if assertions are weak. Mutation testing addresses this by injecting small bugs (mutants) into source code and checking whether tests catch them. Using Infection PHP, a practical walkthrough shows how a tiered pricing service with 100% coverage had an MSI of only 61% due to missing boundary tests. After adding boundary assertions, MSI jumped to 94%. The post covers installing and configuring Infection, setting up CI with GitHub Actions, using --git-diff-filter for PR-scoped runs, and handling common objections like slow execution and equivalent mutants.
Table of contents
The Dirty Secret of Code CoverageWhat Is Mutation Testing?Getting Started with Infection PHPA Real-World Example: Catching What Coverage MissesConfiguring Infection for Your ProjectIntegrating Into CI (GitHub Actions)Common Objections — And Honest AnswersWhat My Workflow Looks Like in 2026Final ThoughtsSort: