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Code Coverage

testing PHP 5.0+ Beginner

Also Known As

test coverage line coverage branch coverage

TL;DR

A metric measuring the percentage of code executed by a test suite — useful for identifying untested paths, not a quality guarantee.

Explanation

Code coverage (line, branch, path, mutation) measures which code was executed during tests. PHPUnit integrates with Xdebug or PCOV to generate coverage reports. High coverage does not guarantee correctness — tests can execute code without asserting meaningful outcomes. Mutation testing (Infection PHP) goes further, introducing bugs into the code and checking whether tests fail. Rather than chasing 100% line coverage, focus coverage efforts on business logic, security-sensitive code, and complex conditionals where untested branches pose real risk.

Diagram

flowchart TD
    subgraph Coverage_Types
        LINE[Line coverage<br/>was each line executed]
        BRANCH[Branch coverage<br/>was each if/else path taken]
        MUT2[Mutation coverage<br/>do tests catch changes]
    end
    subgraph Pitfalls
        FALSE_100[100pct line coverage<br/>but no assertions<br/>tests nothing]
        METRIC[Coverage as a target<br/>Goodharts Law - gaming it]
    end
    subgraph Better_Goal
        MEANINGFUL[Meaningful assertions<br/>test behaviours not lines]
        MUTATION[High mutation score<br/>tests actually catch bugs]
    end
style FALSE_100 fill:#f85149,color:#fff
style METRIC fill:#f85149,color:#fff
style MEANINGFUL fill:#238636,color:#fff
style MUTATION fill:#238636,color:#fff

Common Misconception

✗ 100% code coverage means the code is fully tested. Coverage measures which lines execute during tests, not whether the tests assert meaningful outcomes. A test that calls every line without asserting anything achieves 100% coverage and proves nothing.

Why It Matters

Coverage metrics show which code is exercised by tests — they reveal untested paths and give a floor for confidence. However, 100% coverage does not mean the code is correct, only that every line was executed at least once.

Common Mistakes

Setting 100% coverage as a target — teams write meaningless tests just to hit the number.
Counting coverage on generated code, migrations, and config files — only measure what matters.
Using line coverage instead of branch coverage — a line with an if can be covered without testing the false path.
Treating coverage as sufficient QA — coverage measures quantity of testing, not quality.

Code Examples

💡 Note100% coverage is not the goal — meaningful tests covering real behaviour are. A poorly written test can hit a line without actually asserting anything.

✗ Vulnerable

// Test that covers code without testing behaviour:
public function testUserCreation(): void {
    $user = new User('Alice', 'alice@example.com');
    $this->assertInstanceOf(User::class, $user); // Just checks the object was created
    // 100% line coverage — zero meaningful assertion about User's behaviour
}

✓ Fixed

# phpunit.xml — require 80% line coverage on CI
<coverage>
  <report>
    <clover outputFile='build/coverage.xml'/>
    <html outputDirectory='build/coverage-html'/>
  </report>
</coverage>

# Enforce minimum in CI
$ phpunit --coverage-clover=coverage.xml
$ php vendor/bin/coverage-check coverage.xml 80

References

Tags

testing metrics quality

Added 15 Mar 2026

Edited 22 Mar 2026

Curated in Warsaw under one editorial standard. 1,444 terms, single voice. About this reference →

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Also referenced

Static Analysis 32 Test-Driven Development (TDD) 29 Unit Testing 27

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Related categories

testing 2.5k general 1.4k

⚡ DEV INTEL Tools & Severity

🟡 Medium ⚙ Fix effort: Medium

⚡ Quick Fix

Use PHPUnit with Xdebug or PCOV for coverage — aim for 80% line coverage on business logic; use mutation testing (Infection) to verify the coverage is meaningful

📦 Applies To

PHP 5.0+ web cli queue-worker

🔗 Prerequisites

Unit Testing Test Coverage Types PHPStan Levels 0–9 Explained

🔍 Detection Hints

0% test coverage on critical payment or auth code; coverage target met with assertions that don't verify behaviour; no mutation testing

Auto-detectable: ✓ Yes phpunit xdebug pcov infection codecov

⚠ Related Problems

Test Coverage Types Unit Testing Mutation Testing

🤖 AI Agent

Confidence: Medium False Positives: Medium ✗ Manual fix Fix: Medium Context: File Tests: Update