What is the difference between the 'adapter' and 'decorator' patterns?

Adapter transforms the interface of one class into the interface expected by another class, providing compatibility between classes with incompatible interfaces. It changes the signature of existing methods to match the new interface.

Decorator dynamically adds new responsibilities to an object by wrapping it in a decorator. It does not change the interface of the decorated object but only extends its functionality.

Criterion	Adapter	Decorator
Purpose	Interface compatibility	Functionality extension
Interface change	Yes, transforms one into another	No, preserves the existing
Structure	Wraps the adaptable object	Wraps the decorated object
Behavior	Changes calls to the adaptable object	Adds new actions or modifies existing ones
Example	Converting XML to JSON	Adding logging to an existing class

<?php

// Adapter example
interface TargetInterface {
    public function request();
}

class Adaptee {
    public function specificRequest() {
        echo "Specific request of Adaptee" . PHP_EOL;
    }
}

class Adapter implements TargetInterface {
    private $adaptee;

    public function __construct(Adaptee $adaptee) {
        $this->adaptee = $adaptee;
    }

    public function request() {
        // Transforming the call
        $this->adaptee->specificRequest();
    }
}

// Decorator example
interface Component {
    public function operation();
}

class ConcreteComponent implements Component {
    public function operation() {
        echo "Concrete component operation" . PHP_EOL;
    }
}

class Decorator implements Component {
    protected $component;

    public function __construct(Component $component) {
        $this->component = $component;
    }

    public function operation() {
        $this->component->operation();
    }
}

class ConcreteDecoratorA extends Decorator {
    public function operation() {
        parent::operation();
        $this->addedBehaviorA();
    }

    private function addedBehaviorA() {
        echo "Added behavior A" . PHP_EOL;
    }
}