Strategy pattern in PHP - Mini Project: Build & Apply

Implementing the Strategy Pattern in PHP

📖 Scenario: Imagine you are building a payment system for an online store. The store wants to support multiple payment methods like credit card and PayPal. Each payment method has a different way to process payments.

🎯 Goal: You will create a simple PHP program that uses the Strategy pattern to switch between different payment methods easily. This will help the store process payments using different strategies without changing the main code.

📋 What You'll Learn

Create an interface for payment methods

Implement two payment method classes: CreditCardPayment and PayPalPayment

Create a PaymentContext class that uses a payment strategy

Demonstrate switching payment methods and processing payments

💡 Why This Matters

🌍 Real World

The Strategy pattern is used in software where you want to switch between different algorithms or behaviors easily, like payment processing, sorting methods, or logging strategies.

💼 Career

Understanding design patterns like Strategy is important for writing flexible and maintainable code, a key skill for software developers and engineers.

1

Create the PaymentMethod interface

Create an interface called PaymentMethod with a public method pay that accepts a parameter amount.

PHP

# Your code here

Hint

Use the interface keyword to define the PaymentMethod interface with the pay method.

2

Implement CreditCardPayment and PayPalPayment classes

Create two classes called CreditCardPayment and PayPalPayment that implement the PaymentMethod interface. Each class should have a pay method that prints a message showing the payment method and the amount paid.

PHP

<?php
interface PaymentMethod {
    public function pay(float $amount): void;
}

# Your code here

Hint

Implement the pay method in both classes to print the payment details.

3

Create the PaymentContext class

Create a class called PaymentContext with a private property paymentMethod of type PaymentMethod. Add a constructor that accepts a PaymentMethod and assigns it to paymentMethod. Add a public method setPaymentMethod to change the payment method. Add a public method pay that accepts an amount and calls the pay method of the current paymentMethod.

PHP

<?php
interface PaymentMethod {
    public function pay(float $amount): void;
}

class CreditCardPayment implements PaymentMethod {
    public function pay(float $amount): void {
        echo "Paid $amount using Credit Card.\n";
    }
}

class PayPalPayment implements PaymentMethod {
    public function pay(float $amount): void {
        echo "Paid $amount using PayPal.\n";
    }
}

# Your code here

Hint

Use a private property to hold the current payment method and methods to set it and pay.

4

Use PaymentContext to process payments

Create a PaymentContext object with CreditCardPayment as the initial payment method. Call the pay method with amount 100.0. Then change the payment method to PayPalPayment using setPaymentMethod and call pay again with amount 200.0. Print the output.

PHP

paymentMethod = $paymentMethod;
    }

    public function setPaymentMethod(PaymentMethod $paymentMethod): void {
        $this->paymentMethod = $paymentMethod;
    }

    public function pay(float $amount): void {
        $this->paymentMethod->pay($amount);
    }
}

# Your code here

Hint

Create the context with CreditCardPayment, pay 100, switch to PayPalPayment, then pay 200.

Practice

1.

What is the main purpose of the Strategy pattern in PHP?

easy

A. To allow changing the algorithm or behavior at runtime without modifying the context.

B. To create multiple instances of the same class.

C. To store data in a database efficiently.

D. To handle errors and exceptions in the code.

Solution

1. Step 1: Understand the Strategy pattern concept

   The Strategy pattern is designed to let you swap different algorithms or behaviors easily without changing the main code that uses them.

2. Step 2: Identify the correct purpose

   Among the options, only To allow changing the algorithm or behavior at runtime without modifying the context. describes this behavior: changing the algorithm at runtime without modifying the context class.

3. Final Answer:

   To allow changing the algorithm or behavior at runtime without modifying the context. -> Option A

4. Quick Check:

   Strategy pattern = change behavior easily [OK]

Hint: Strategy pattern swaps algorithms without changing main code [OK]

Common Mistakes:

• Confusing Strategy with Singleton or Factory patterns
• Thinking it manages data storage
• Assuming it handles errors

2.

Which of the following is the correct way to define a strategy interface in PHP?

?

easy

A. interface Strategy { function execute() {} }

B. class Strategy { function execute() {} }

C. interface Strategy { public function execute(); }

D. abstract class Strategy { public execute(); }

Solution

1. Step 1: Recall PHP interface syntax

   In PHP, interfaces are declared with the keyword interface, and methods inside must be declared without a body and with visibility (usually public).

2. Step 2: Check each option

   interface Strategy { public function execute(); } correctly declares an interface with a public method without a body. class Strategy { function execute() {} } uses a class, not interface. interface Strategy { function execute() {} } incorrectly provides a method body inside interface. abstract class Strategy { public execute(); } uses abstract class but misses parentheses and visibility.

3. Final Answer:

   interface Strategy { public function execute(); } -> Option C

4. Quick Check:

   Interface method = declaration only [OK]

Hint: Interface methods have no body, use 'public function name();' [OK]

Common Mistakes:

• Adding method body inside interface
• Using class instead of interface
• Omitting visibility keyword

3.

What will be the output of the following PHP code using the Strategy pattern?

interface Strategy {
    public function execute();
}

class AddStrategy implements Strategy {
    public function execute() {
        return 5 + 3;
    }
}

class MultiplyStrategy implements Strategy {
    public function execute() {
        return 5 * 3;
    }
}

class Context {
    private Strategy $strategy;
    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }
    public function executeStrategy() {
        return $this->strategy->execute();
    }
}

$context = new Context(new MultiplyStrategy());
echo $context->executeStrategy();

medium

A. 8

B. 15

C. 53

D. Error: Cannot instantiate interface

Solution

1. Step 1: Understand the code flow

   The code defines two strategies: AddStrategy returns 5+3=8, MultiplyStrategy returns 5*3=15. The Context class uses the strategy passed in constructor.

2. Step 2: Identify which strategy is used

   The context is created with new MultiplyStrategy(), so calling executeStrategy() returns 15.

3. Final Answer:

   15 -> Option B

4. Quick Check:

   MultiplyStrategy returns 15 [OK]

Hint: Check which strategy object is passed to context [OK]

Common Mistakes:

• Confusing AddStrategy and MultiplyStrategy output
• Thinking interface cannot be instantiated (only concrete classes)
• Ignoring which strategy is passed

4.

Identify the error in the following PHP code implementing the Strategy pattern:

interface Strategy {
    public function execute();
}

class ConcreteStrategy implements Strategy {
    public function execute() {
        echo "Running strategy";
    }
}

class Context {
    private $strategy;
    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }
    public function executeStrategy() {
        $this->strategy->execute;
    }
}

$context = new Context(new ConcreteStrategy());
$context->executeStrategy();

medium

A. Cannot instantiate interface directly

B. Strategy interface missing method body

C. Context constructor missing type hint

D. Missing parentheses when calling execute method

Solution

1. Step 1: Check method call syntax in Context

   Inside executeStrategy(), the code calls $this->strategy->execute; without parentheses, so the method is not actually called.

2. Step 2: Identify correct fix

   Method calls require parentheses even if no arguments: $this->strategy->execute();.

3. Final Answer:

   Missing parentheses when calling execute method -> Option D

4. Quick Check:

   Method call needs () [OK]

Hint: Always use () to call methods in PHP [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Confusing interface method declaration with implementation
• Ignoring type hints in constructor

5.

You want to add a new strategy SubtractStrategy to the existing PHP Strategy pattern code. Which of the following is the best way to implement it?

interface Strategy {
    public function execute(int $a, int $b): int;
}

class AddStrategy implements Strategy {
    public function execute(int $a, int $b): int {
        return $a + $b;
    }
}

// Your code here

hard

A. class SubtractStrategy implements Strategy {
public function execute(int $a, int $b): int {
return $a - $b;
}
}

B. class SubtractStrategy {
public function execute(int $a, int $b): int {
return $a - $b;
}
}

C. class SubtractStrategy implements Strategy {
public function execute(int $a, int $b) {
return $a - $b;
}
}

D. class SubtractStrategy implements Strategy {
public function execute($a, $b): int {
return $a - $b;
}
}

Solution

1. Step 1: Check interface method signature

   The interface requires execute(int $a, int $b): int with type hints and return type.

2. Step 2: Match method signature exactly in SubtractStrategy

   class SubtractStrategy implements Strategy { public function execute(int $a, int $b): int { return $a - $b; } } matches the interface exactly with parameter types and return type. class SubtractStrategy { public function execute(int $a, int $b): int { return $a - $b; } } misses implementing the interface. class SubtractStrategy implements Strategy { public function execute(int $a, int $b) { return $a - $b; } } misses return type. class SubtractStrategy implements Strategy { public function execute($a, $b): int { return $a - $b; } } misses parameter types.

3. Final Answer:

   class SubtractStrategy implements Strategy { public function execute(int $a, int $b): int { return $a - $b; } } -> Option A

4. Quick Check:

   Implement interface with exact method signature [OK]

Hint: Match interface method signature exactly when implementing [OK]

Common Mistakes:

• Omitting interface implementation
• Missing type hints or return types
• Changing method signature