Behaviour-Driven Development (BDD)

Also Known As

TL;DR

Explanation

BDD extends TDD by making tests readable to non-technical stakeholders. The Given-When-Then structure (Gherkin syntax) describes behaviour: Given a context, When an action occurs, Then an outcome is expected. In PHP, Behat is the primary BDD framework. The value is not the syntax but the conversation it forces — writing in Gherkin requires alignment on what the system should actually do before anyone writes code.

Diagram

sequenceDiagram
    participant PO as Product Owner
    participant DEV as Developer
    participant QA as QA
    PO->>DEV: User story
    DEV->>DEV: Write Gherkin scenario
    Note over DEV: Given user is logged in<br/>When they add item to cart<br/>Then cart count increases
    DEV->>QA: Review scenario
    QA-->>DEV: Approved
    DEV->>DEV: Implement step definitions
    DEV->>DEV: Run Behat - fails
    DEV->>DEV: Implement feature
    DEV->>DEV: Run Behat - passes
    Note over DEV,QA: Scenario = living documentation

Common Misconception

Why It Matters

Common Mistakes

• Writing BDD scenarios for technical implementation details instead of business behaviour.
• Not involving domain experts in writing scenarios — Gherkin written only by developers defeats the purpose.
• Too many steps in a scenario — each scenario should test one behaviour; multiple behaviours need multiple scenarios.
• Using BDD for unit tests — BDD is for business-facing acceptance criteria, not internal implementation testing.

Code Examples

# BDD scenario testing implementation, not behaviour:
Scenario: Test the UserRepository::findByEmail method
    Given the database has a users table
    When I call UserRepository::findByEmail('alice@example.com')
    Then the PDOStatement should return one row
    # This tests a method, not a business behaviour

# BDD scenario testing business behaviour:
Feature: User authentication
  Scenario: Registered user logs in with correct credentials
    Given a user exists with email 'alice@example.com' and password 'secret'
    When Alice submits the login form with correct credentials
    Then she should be redirected to the dashboard
    And she should see a welcome message

  Scenario: Wrong password is rejected
    Given a user exists with email 'alice@example.com'
    When Alice submits the login form with the wrong password
    Then she should see an error message
    And she should remain on the login page

References

• ↗ https://behat.org/
• ↗ https://en.wikipedia.org/wiki/Behavior-driven_development

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