Mutation Testing with Infection

100% line coverage feels reassuring, but it only proves your tests executed the code — not that they would catch a bug in it. A test can run a line and assert nothing meaningful. Mutation testing measures the real thing: would your tests fail if the code were subtly broken? Infection is the standard PHP tool for this.

Infection takes your covered code and introduces tiny faults — mutants. It flips > to >=, + to -, && to ||, removes a return, etc. Then it re-runs your tests against each mutant:

• If a test fails → the mutant is killed (good — your tests caught the fault).
• If all tests pass → the mutant survived (bad — a real bug here would go unnoticed).