Strategy pattern in PHP - Step-by-Step Execution

Concept Flow - Strategy pattern

Client code

↓

Choose strategy

↓

Set strategy object

↓

Call strategy method

↓

Strategy executes algorithm

↓

Return result to client

The client picks a strategy object, sets it, then calls its method to run the chosen algorithm.

Execution Sample

PHP

<?php
interface Strategy {
  public function doOperation(int $a, int $b): int;
}

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

class Context {
  private Strategy $strategy;
  public function __construct(Strategy $strategy) {
    $this->strategy = $strategy;
  }
  public function executeStrategy(int $a, int $b): int {
    return $this->strategy->doOperation($a, $b);
  }
}

$addStrategy = new AddStrategy();
$context = new Context($addStrategy);
echo $context->executeStrategy(5, 3); // Outputs 8
?>

This code uses the Strategy pattern to add two numbers by choosing the AddStrategy.

Execution Table

Step	Action	Object/Variable	Value/Result	Explanation
1	Create AddStrategy object	$addStrategy	AddStrategy instance	AddStrategy object created to perform addition
2	Create Context with AddStrategy	$context	Context instance with AddStrategy	Context stores the strategy object
3	Call executeStrategy(5, 3)	executeStrategy	Calls AddStrategy->doOperation(5,3)	Context delegates operation to strategy
4	AddStrategy->doOperation(5,3)	doOperation	8	AddStrategy adds 5 + 3 and returns 8
5	executeStrategy returns	executeStrategy	8	Context returns the result from strategy
6	echo output	Output	8	Program prints 8 to the screen

💡 Execution ends after printing the result 8

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
$addStrategy	null	AddStrategy instance	AddStrategy instance	AddStrategy instance	AddStrategy instance	AddStrategy instance
$context	null	null	Context instance with AddStrategy	Context instance with AddStrategy	Context instance with AddStrategy	Context instance with AddStrategy
Result	null	null	null	null	8	8

Key Moments - 3 Insights

Why does the Context class need a Strategy object?

The Context holds a Strategy object to delegate the operation. As shown in step 2 and 3 of the execution_table, Context calls the strategy's method to perform the algorithm.

How does the program decide which algorithm to run?

The program decides by which Strategy object is passed to Context. In step 1, AddStrategy is created and passed to Context in step 2, so addition is performed.

What if we want to change the operation at runtime?

We can create a new Strategy object and set it in Context. This flexibility is the main benefit of the Strategy pattern, allowing different algorithms without changing Context.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value returned by AddStrategy->doOperation at step 4?

A 3

B 5

C 8

D null

💡 Hint

Check the 'Value/Result' column in row for step 4 in execution_table

At which step does the Context object get created with the strategy?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Object/Variable' columns in execution_table for when Context is created

If we replaced AddStrategy with a MultiplyStrategy, what would change in the execution_table?

A Step 4 result would be multiplication of inputs

B Step 2 would not create Context

C Step 6 would print null

D No change at all

💡 Hint

Focus on step 4 where the strategy's operation result is shown

Concept Snapshot

Strategy pattern lets you choose an algorithm at runtime.
Define a Strategy interface with a method.
Create concrete Strategy classes implementing the interface.
Context holds a Strategy object and calls its method.
Change strategy to change behavior without changing Context.

Full Transcript

The Strategy pattern helps us pick different ways to do a task without changing the main code. We create a Strategy interface with a method for the task. Then, we make classes that do the task differently. The Context class holds one of these strategy objects and calls its method to run the chosen algorithm. In the example, AddStrategy adds two numbers. The Context uses AddStrategy to add 5 and 3, returning 8. This way, we can swap strategies anytime to change behavior easily.

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