PHP Design Patterns

Before writing code

Fetch live docs: Web-search php design patterns modern for current pattern implementations. Reference https://refactoring.guru/design-patterns/php for comprehensive examples.

Singleton

What It Does

Ensures only one instance of a class exists. Common in WordPress/WooCommerce plugins as the main plugin class.

WooCommerce Usage

WC() returns the single WooCommerce instance. Extension main classes typically follow:

• Private __construct(), private __clone(), private __wakeup()
• Static instance() method
• Global accessor function (e.g., function my_extension() { return My_Extension::instance(); })

When to Use

Plugin entry points only. Avoid for everything else — singletons are hard to test and create hidden dependencies.

Factory

What It Does

Creates objects without specifying the exact class. Decouples object creation from usage.

WooCommerce Usage

WC_Product_Factory resolves product type to correct class:

• wc_get_product( $id ) — factory method returns WC_Product_Simple, WC_Product_Variable, etc.
• Filterable via woocommerce_product_class hook

Implementation

Define a factory method or class that maps identifiers to concrete classes. Use for: product types, payment gateway selection, shipping method resolution.

Strategy

What It Does

Defines a family of interchangeable algorithms. Client code works with the interface, not the implementation.

WooCommerce Usage

• Payment gateways — all implement WC_Payment_Gateway, swappable at checkout
• Shipping methods — all extend WC_Shipping_Method, configurable per zone
• Tax calculators — different calculation strategies per location

Implementation

Define an interface/abstract class. Create concrete implementations. Select the strategy at runtime based on configuration or context.

Observer (Hooks)

What It Does

Objects subscribe to events and get notified when they occur. Loose coupling between event producers and consumers.

WooCommerce Usage

WordPress hooks ARE the Observer pattern:

• do_action() / apply_filters() — notify observers
• add_action() / add_filter() — subscribe
• The entire WordPress/WooCommerce extensibility model is built on this

Implementation

Use WordPress hooks for WooCommerce extensions. For internal events within your plugin, consider firing custom actions: do_action( 'my_plugin_after_process', $data ).

Repository

What It Does

Mediates between the domain layer and data mapping layer. Provides collection-like access to data.

WooCommerce Usage

While WooCommerce uses Data Stores rather than formal Repositories, the concept is similar:

• wc_get_orders( $args ) — query-based data access
• wc_get_products( $args ) — product collection access
• CRUD objects encapsulate persistence

Implementation

Create a class that provides methods like find_by_id(), find_all(), save(), delete(). Internally use wc_get_orders() or custom queries.

Decorator

What It Does

Adds behavior to objects dynamically by wrapping them. Each decorator adds one concern.

WooCommerce Usage

Filters act as decorators:

• apply_filters( 'woocommerce_product_get_price', $price, $product ) — each filter wraps/modifies the value
• Stacked filters effectively decorate the data pipeline

Implementation

Create wrapper classes that implement the same interface as the wrapped object. Delegate most calls, add behavior where needed.

Dependency Injection

What It Does

Objects receive their dependencies through constructors or setters rather than creating them internally.

WooCommerce/WordPress Context

WordPress doesn't have a built-in DI container, but you can practice DI:

• Pass dependencies through constructors
• Use a lightweight container (PHP-DI, League Container) for larger extensions
• Avoid static/global state — inject WC(), $wpdb, etc. where possible

Implementation

class OrderProcessor {
    public function __construct(
        private readonly PaymentGateway $gateway,
        private readonly ShippingCalculator $shipping,
    ) {}
}

Service Locator

What It Does

A registry that provides access to services. Less preferred than DI, but practical in WordPress.

WooCommerce Usage

• WC()->payment_gateways() — access payment gateways registry
• WC()->shipping() — access shipping methods registry
• WC()->cart — access cart instance

Template Method

What It Does

Defines the skeleton of an algorithm in a base class, letting subclasses override specific steps.

WooCommerce Usage

• WC_Payment_Gateway — defines the gateway lifecycle; subclasses implement process_payment(), init_form_fields()
• WC_Shipping_Method — defines shipping lifecycle; subclasses implement calculate_shipping()

Best Practices

• Use Singleton sparingly — only for plugin main class
• Prefer composition over inheritance
• Use WordPress hooks (Observer pattern) for extensibility
• Inject dependencies through constructors when practical
• Use Factory pattern for polymorphic object creation
• Apply SOLID principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion
• Keep classes small and focused

Fetch design pattern references for current PHP implementations and WooCommerce core examples before implementing.