Strategy pattern in PHP

Introduction

The Strategy pattern helps you change how a program works by swapping different methods easily. It keeps your code clean and flexible.

When you want to choose between different ways to do a task while the program runs.

When you want to avoid big if-else or switch statements deciding what to do.

When you want to add new ways to do something without changing existing code.

When different parts of your program need to do the same job in different ways.

When you want to test different behaviors separately.

Syntax

PHP

<?php

interface Strategy {
    public function execute(string $data): string;
}

class ConcreteStrategyA implements Strategy {
    public function execute(string $data): string {
        return strtoupper($data);
    }
}

class ConcreteStrategyB implements Strategy {
    public function execute(string $data): string {
        return strtolower($data);
    }
}

class Context {
    private Strategy $strategy;

    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }

    public function setStrategy(Strategy $strategy): void {
        $this->strategy = $strategy;
    }

    public function doAction(string $data): string {
        return $this->strategy->execute($data);
    }
}

The Strategy interface defines a method all strategies must implement.

The Context class uses a strategy object to perform its task.

Examples

This example shows two strategies: one to make text uppercase, another lowercase. The context uses the uppercase strategy first.

PHP

<?php

interface Strategy {
    public function execute(string $data): string;
}

class UpperCaseStrategy implements Strategy {
    public function execute(string $data): string {
        return strtoupper($data);
    }
}

class LowerCaseStrategy implements Strategy {
    public function execute(string $data): string {
        return strtolower($data);
    }
}

class Context {
    private Strategy $strategy;

    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }

    public function doAction(string $data): string {
        return $this->strategy->execute($data);
    }
}

$context = new Context(new UpperCaseStrategy());
echo $context->doAction("Hello World");

Here, the context switches to the lowercase strategy and processes the same text differently.

PHP

<?php

$context = new Context(new LowerCaseStrategy());
echo $context->doAction("Hello World");

Sample Program

This program shows how the context uses two different strategies to change the text. First, it makes the text uppercase, then lowercase.

PHP

<?php

interface Strategy {
    public function execute(string $data): string;
}

class UpperCaseStrategy implements Strategy {
    public function execute(string $data): string {
        return strtoupper($data);
    }
}

class LowerCaseStrategy implements Strategy {
    public function execute(string $data): string {
        return strtolower($data);
    }
}

class Context {
    private Strategy $strategy;

    public function __construct(Strategy $strategy) {
        $this->strategy = $strategy;
    }

    public function setStrategy(Strategy $strategy): void {
        $this->strategy = $strategy;
    }

    public function doAction(string $data): string {
        return $this->strategy->execute($data);
    }
}

$context = new Context(new UpperCaseStrategy());
echo $context->doAction("Hello Strategy Pattern!") . "\n";

$context->setStrategy(new LowerCaseStrategy());
echo $context->doAction("Hello Strategy Pattern!") . "\n";

Important Notes

Each strategy class must follow the same interface so the context can use them interchangeably.

You can add new strategies without changing the context code.

Use the Strategy pattern to keep your code easy to read and maintain.

Summary

The Strategy pattern lets you swap different ways to do a task easily.

It helps avoid complex if-else statements by using separate classes.

The context class uses a strategy object to perform actions.

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