Golden Master and Approval Tests

When code produces large outputs (HTML pages, PDF data, CSV exports), a golden master (approval) test runs it with fixed inputs, stores the output as an approved file, and compares future runs against it. Libraries like spatie/phpunit-snapshot-assertions automate this. They are coarse but quickly give a safety net over code you don't understand yet.

<?php
use PHPUnit\Framework\TestCase;
use Spatie\Snapshots\MatchesSnapshots;

final class InvoiceRenderTest extends TestCase
{
    use MatchesSnapshots;

    public function testInvoiceHtmlIsUnchanged(): void
    {
        $invoice = InvoiceFixtures::paidInvoiceWithThreeLines();   // deterministic data
        $html = render_invoice_html($invoice, new DateTimeImmutable('2024-01-15'));
        $this->assertMatchesHtmlSnapshot($html);
    }
}

Common pitfalls

• Non-deterministic output (timestamps, random IDs) making snapshots flaky — inject fixed values.
• Blindly updating snapshots when tests fail.
• Relying only on snapshots long-term instead of focused tests.

Key points

• Snapshots give a fast safety net
• Make outputs deterministic
• Review snapshot diffs carefully

Further reading: spatie/phpunit-snapshot-assertions