Frontend/Backend Parity: Treat Your UI Logic as the Specification

When duplicating complex UI validation or calculation logic on the backend, treat the frontend component as the source of truth. Copy the logic line-by-line into your PHP services, and include comments with the original file and line numbers.

// Replicating logic from ProductForm.vue:142
if ($data['price'] > 0) {
    // ...
}

This makes future parity checks easier and ensures your backend handles edge cases exactly as the user experience defines them. It also serves as great documentation for why certain backend checks exist.