OOP and Classes

Object-oriented programming (OOP) is a paradigm that organizes code around objects rather than functions and procedures. PHP has robust OOP support, and most modern PHP frameworks and libraries are built entirely with classes and objects. In the previous lesson on arrays and objects, you got a brief introduction. Now it is time to go deeper into the full power of PHP's object-oriented features.

What You'll Learn

How to define classes with typed properties
Constructors and constructor promotion
Visibility modifiers: public, private, and protected
Inheritance and method overriding
Abstract classes and interfaces
Static methods and properties
Readonly properties in PHP 8

Classes and Objects

A class is a blueprint for creating objects. It defines properties (data) and methods (behavior):

<?php

class Product {
    public string $name;
    public float $price;
    public int $quantity;

    public function __construct(string $name, float $price, int $quantity = 0) {
        $this->name = $name;
        $this->price = $price;
        $this->quantity = $quantity;
    }

    public function totalValue(): float {
        return $this->price * $this->quantity;
    }

    public function describe(): string {
        return "{$this->name}: \${$this->price} x {$this->quantity} = \${$this->totalValue()}";
    }
}

$laptop = new Product("Laptop", 999.99, 5);
$mouse = new Product("Mouse", 29.99, 50);

echo $laptop->describe() . "\n";
echo $mouse->describe() . "\n";
echo "Laptop stock value: $" . $laptop->totalValue() . "\n";

The __construct method is a special method called automatically when you create a new object with new. Inside the class, $this refers to the current object instance.

Constructor Promotion

PHP 8 introduced constructor promotion, which lets you declare and assign properties in a single line. This eliminates a lot of boilerplate code:

<?php

class User {
    // Constructor promotion: declare and assign in one step
    public function __construct(
        public readonly string $name,
        public string $email,
        private string $password,
        public string $role = 'user'
    ) {}

    public function getInfo(): string {
        return "{$this->name} ({$this->email}) - Role: {$this->role}";
    }

    public function verifyPassword(string $attempt): bool {
        return $this->password === $attempt;
    }
}

$admin = new User("Alice", "alice@example.com", "secret123", "admin");
$user = new User("Bob", "bob@example.com", "pass456");

echo $admin->getInfo() . "\n";
echo $user->getInfo() . "\n";

// Readonly properties cannot be changed after construction
// $admin->name = "Eve"; // This would throw an error

echo "Password check: " . ($admin->verifyPassword("secret123") ? "Valid" : "Invalid") . "\n";
echo "Password check: " . ($admin->verifyPassword("wrong") ? "Valid" : "Invalid") . "\n";

With constructor promotion, the public, private, or protected keyword before a constructor parameter automatically creates and assigns a class property. The readonly modifier (PHP 8.1) makes the property immutable after construction.

Visibility Modifiers

Visibility controls who can access properties and methods. PHP has three levels:

public -- accessible from anywhere
protected -- accessible within the class and its children
private -- accessible only within the class itself

<?php

class BankAccount {
    private float $balance;
    private array $transactions = [];

    public function __construct(
        public readonly string $accountNumber,
        public readonly string $owner,
        float $initialBalance = 0.0
    ) {
        $this->balance = $initialBalance;
        $this->logTransaction("Account opened", $initialBalance);
    }

    public function deposit(float $amount): void {
        if ($amount <= 0) {
            throw new InvalidArgumentException("Deposit must be positive");
        }
        $this->balance += $amount;
        $this->logTransaction("Deposit", $amount);
    }

    public function withdraw(float $amount): void {
        if ($amount <= 0) {
            throw new InvalidArgumentException("Withdrawal must be positive");
        }
        if ($amount > $this->balance) {
            throw new RuntimeException("Insufficient funds");
        }
        $this->balance -= $amount;
        $this->logTransaction("Withdrawal", -$amount);
    }

    public function getBalance(): float {
        return $this->balance;
    }

    private function logTransaction(string $type, float $amount): void {
        $this->transactions[] = [
            'type' => $type,
            'amount' => $amount,
            'date' => date('Y-m-d H:i:s'),
        ];
    }

    public function getTransactionCount(): int {
        return count($this->transactions);
    }
}

$account = new BankAccount("ACC-001", "Alice", 1000.00);
$account->deposit(500.00);
$account->withdraw(200.00);

echo "Owner: {$account->owner}\n";
echo "Balance: $" . $account->getBalance() . "\n";
echo "Transactions: " . $account->getTransactionCount() . "\n";

// $account->balance; // Error: private property
// $account->logTransaction(...); // Error: private method

Notice that $balance and $transactions are private, so outside code cannot directly change the balance. The class controls access through public methods like deposit() and withdraw(), ensuring business rules are enforced.

Inheritance

Inheritance lets a child class extend a parent class, reusing and overriding behavior:

<?php

class Shape {
    public function __construct(
        protected string $color = "red"
    ) {}

    public function getColor(): string {
        return $this->color;
    }

    public function area(): float {
        return 0.0;
    }

    public function describe(): string {
        return "A {$this->color} shape with area " . round($this->area(), 2);
    }
}

class Circle extends Shape {
    public function __construct(
        private float $radius,
        string $color = "red"
    ) {
        parent::__construct($color);
    }

    public function area(): float {
        return M_PI * $this->radius ** 2;
    }

    public function describe(): string {
        return "A {$this->color} circle (r={$this->radius}) with area " . round($this->area(), 2);
    }
}

class Rectangle extends Shape {
    public function __construct(
        private float $width,
        private float $height,
        string $color = "blue"
    ) {
        parent::__construct($color);
    }

    public function area(): float {
        return $this->width * $this->height;
    }

    public function describe(): string {
        return "A {$this->color} rectangle ({$this->width}x{$this->height}) with area " . round($this->area(), 2);
    }
}

$shapes = [
    new Circle(5.0, "green"),
    new Rectangle(4.0, 6.0),
    new Circle(3.0),
];

foreach ($shapes as $shape) {
    echo $shape->describe() . "\n";
}

The extends keyword creates a child class. The child inherits all public and protected members of the parent. Use parent::__construct() to call the parent's constructor. Overriding a method replaces the parent's implementation in the child.

Abstract Classes and Interfaces

Abstract classes define methods that child classes must implement. Interfaces define a contract that implementing classes must fulfill:

<?php

interface Printable {
    public function toString(): string;
}

abstract class Animal implements Printable {
    public function __construct(
        protected string $name,
        protected int $age
    ) {}

    // Abstract method: children MUST implement this
    abstract public function speak(): string;

    // Concrete method: children inherit this
    public function describe(): string {
        return "{$this->name} is {$this->age} years old";
    }

    public function toString(): string {
        return "{$this->name} says: {$this->speak()}";
    }
}

class Dog extends Animal {
    public function speak(): string {
        return "Woof!";
    }

    public function fetch(string $item): string {
        return "{$this->name} fetches the {$item}!";
    }
}

class Cat extends Animal {
    public function speak(): string {
        return "Meow!";
    }

    public function purr(): string {
        return "{$this->name} purrs contentedly.";
    }
}

$dog = new Dog("Rex", 3);
$cat = new Cat("Whiskers", 5);

echo $dog->describe() . "\n";
echo $dog->toString() . "\n";
echo $dog->fetch("ball") . "\n";

echo $cat->describe() . "\n";
echo $cat->toString() . "\n";
echo $cat->purr() . "\n";

// You cannot instantiate an abstract class:
// $animal = new Animal("Test", 1); // Error!

Use abstract classes when you want to share common code between related classes. Use interfaces when you want unrelated classes to share a common contract.

Static Methods and Properties

Static members belong to the class itself, not to individual instances:

<?php

class Counter {
    private static int $count = 0;

    public static function increment(): void {
        self::$count++;
    }

    public static function getCount(): int {
        return self::$count;
    }

    public static function reset(): void {
        self::$count = 0;
    }
}

Counter::increment();
Counter::increment();
Counter::increment();
echo "Count: " . Counter::getCount() . "\n";

Counter::reset();
echo "After reset: " . Counter::getCount() . "\n";

// Practical example: a factory method
class DatabaseConnection {
    private static ?DatabaseConnection $instance = null;

    private function __construct(
        private string $host,
        private string $database
    ) {}

    public static function create(string $host, string $database): self {
        if (self::$instance === null) {
            self::$instance = new self($host, $database);
        }
        return self::$instance;
    }

    public function getInfo(): string {
        return "Connected to {$this->database} on {$this->host}";
    }
}

$db = DatabaseConnection::create("localhost", "myapp");
echo $db->getInfo() . "\n";

Static methods are called with ClassName::method() instead of $object->method(). Use self:: to reference static members within the class. The Singleton pattern shown above is a classic use of static properties.

Key Takeaways

Classes define blueprints for objects with properties and methods
Constructor promotion (PHP 8) reduces boilerplate code significantly
Use visibility modifiers to enforce encapsulation: public, protected, private
Inheritance with extends enables code reuse and polymorphism
Abstract classes provide partial implementations; interfaces define contracts
Static members belong to the class, not to individual instances
The readonly modifier (PHP 8.1) creates immutable properties

Next Steps

Now that you understand object-oriented PHP, you are ready to learn about error handling. In the next lesson, you will discover how to use try/catch blocks, create custom exceptions, and write robust code that gracefully handles failures.

Pro Tip: Follow the Single Responsibility Principle -- each class should have one reason to change. If a class is doing too many things (handling HTTP requests, querying the database, and formatting output), split it into smaller focused classes. This makes your code easier to test, understand, and maintain. Start with simple classes and add complexity only when needed.