PHP OOP Fundamentals¶

PHP 8.x OOP covers classes, inheritance, interfaces, abstract classes, traits, namespaces, and PSR-4 autoloading. Everything is private by default in modern PHP. Constructor promotion (PHP 8) eliminates boilerplate. Namespaces prevent naming conflicts; Composer's PSR-4 autoloader maps namespaces to directories.

Key Facts¶

Constructor promotion (PHP 8): public function __construct(private string $name) - declares + assigns
readonly properties (PHP 8.1): set once in constructor, immutable after
Visibility: public, protected, private - default is implicit public for methods
abstract class cannot be instantiated; interface defines contract with no implementation
trait provides horizontal code reuse (multiple traits per class, resolves conflicts)
PSR-4 autoloading: namespace App\Controllers maps to app/Controllers/ directory

Patterns¶

Class with Constructor Promotion¶

class User {
    public function __construct(
        private readonly string $name,
        private string $email,
        private string $role = 'user',
    ) {}

    public function getName(): string { return $this->name; }
    public function getRole(): string { return $this->role; }
}

Inheritance and Abstract Classes¶

abstract class Shape {
    abstract public function area(): float;

    public function describe(): string {
        return static::class . ": area = " . $this->area();
    }
}

class Circle extends Shape {
    public function __construct(private float $radius) {}
    public function area(): float { return M_PI * $this->radius ** 2; }
}

Interfaces¶

interface Renderable {
    public function render(): string;
}

interface Cacheable {
    public function getCacheKey(): string;
    public function getCacheTtl(): int;
}

class Widget implements Renderable, Cacheable {
    public function render(): string { return '<div>Widget</div>'; }
    public function getCacheKey(): string { return 'widget_' . $this->id; }
    public function getCacheTtl(): int { return 3600; }
}

Traits¶

trait Timestampable {
    public ?string $created_at = null;
    public ?string $updated_at = null;

    public function touch(): void {
        $this->updated_at = date('Y-m-d H:i:s');
    }
}

class Post {
    use Timestampable;
}

Namespaces and Autoloading¶

// src/Controllers/UserController.php
namespace App\Controllers;

use App\Models\User;

class UserController {
    public function index(): array {
        return User::all();
    }
}

// composer.json
{
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    }
}

Gotchas¶

static::method() uses late static binding (resolves at runtime); self::method() resolves at definition time
Traits with conflicting method names need explicit resolution: use A, B { A::method insteadof B; }
Constructor in child must call parent::__construct() if parent defines one
readonly properties cannot be modified after initial assignment - even by the object itself