Intro to Design Patterns

Part of the Object Oriented Programming section of Coddy's PHP journey — lesson 70 of 91.

Design patterns are proven solutions to common problems that developers encounter repeatedly when building software. Rather than reinventing the wheel each time, you can apply these established templates to solve similar challenges in your own code.

Think of design patterns as recipes. Just as a chef follows a recipe to create a dish that's been perfected over time, developers use design patterns to structure their code in ways that have been tested and refined by countless programmers before them.

Design patterns are typically grouped into three categories:

Category	Purpose	Examples
Creational	How objects are created	Singleton, Factory
Structural	How objects are composed	Adapter, Decorator
Behavioral	How objects communicate	Observer, Strategy

You've actually been using patterns throughout this course without realizing it. When you created abstract classes with methods that child classes must implement, you were applying a form of the Template Method pattern. When you used interfaces to define contracts, you were setting up for the Strategy pattern.

Learning design patterns gives you a shared vocabulary with other developers. Instead of explaining a complex structure in detail, you can simply say "I used the Factory pattern here" and experienced developers will immediately understand your approach. In the following lessons, we'll implement several essential patterns in PHP.

Challenge

Easy

Let's build a simple message formatting system that demonstrates how design patterns help organize code. You'll identify which pattern category each component belongs to and see how different parts of a system work together.

You'll organize your code across three files:

MessageFormatter.php — Create an interface called MessageFormatter that defines a contract for formatting messages. It should have a single method format(string $message): string. This represents the behavioral category — it defines how objects communicate and process data.
Formatters.php — Include the MessageFormatter file and create two classes that implement the interface:
- UppercaseFormatter — Its format() method returns the message converted to uppercase
- BracketFormatter — Its format() method wraps the message in square brackets like [message]
main.php — Include the Formatters file. You'll receive two inputs: a message string and a formatter type ("upper" or "bracket").
Create a function called processMessage that accepts a MessageFormatter and a message string, then returns the formatted result. This function doesn't need to know which specific formatter it receives — it just calls format() on whatever object is passed in.

Based on the formatter type input, create the appropriate formatter object. Pass it to your processMessage function along with the message, and print the result.

On a new line, print which pattern category this structure represents. Since we're defining interchangeable behaviors through an interface, print: Pattern category: Behavioral

This challenge shows how interfaces let you swap implementations without changing the code that uses them — a foundation for patterns like Strategy that you'll explore in upcoming lessons.

Cheat sheet

Design patterns are proven solutions to common programming problems that provide templates for structuring code effectively.

Design patterns are grouped into three categories:

Category	Purpose	Examples
Creational	How objects are created	Singleton, Factory
Structural	How objects are composed	Adapter, Decorator
Behavioral	How objects communicate	Observer, Strategy

Design patterns provide a shared vocabulary among developers, making it easier to communicate complex architectural decisions.

Try it yourself

<?php
require_once 'Formatters.php';

// Read input
$message = trim(fgets(STDIN));
$formatterType = trim(fgets(STDIN));

// TODO: Create a function called processMessage that accepts a MessageFormatter
// and a message string, then returns the formatted result


// TODO: Based on $formatterType ("upper" or "bracket"), create the appropriate formatter


// TODO: Call processMessage with the formatter and message, then print the result


// TODO: Print the pattern category on a new line
// Format: "Pattern category: Behavioral"

?>

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Intro to Design Patterns

Cheat sheet

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