Property-Based Testing for Domain Rules in PHP

Book: Decoupled PHP — Clean and Hexagonal Architecture for Applications That Outlive the Framework Also by me: Thinking in Go (2-book series) — Complete Guide to Go Programming + Hexagonal Architecture in Go My project: Hermes IDE | GitHub — an IDE for developers who ship with Claude Code and other AI coding tools Me: xgabriel.com | GitHub You write a Money value object. You write a test. The test does this: public function testAddTwoEuros(): void { $a = Money::of(100, 'EUR'); $b = Money::of(200, 'EUR'); self::assertSame(300, $a->add($b)->amount()); } It passes. You ship it. Six months later, a colleague refactors add() to short-circuit when one side is zero, and the test still passes. Then production starts producing receipts where 0 EUR + 50 EUR = 0 EUR, because the early-return path returns $this without copying the other side's amount. The test wasn't wrong. It was insufficient. You checked the one pair of numbers you happened to type