class / new / $this Since: PHP 5（2004）

// Define a class.
class ClassName {
	// Property declaration
	public $property;

	// Constructor
	public function __construct($param) {
		$this->property = $param;
	}

	// Method definition
	public function method() {
		return $this->property;
	}
}

// Create an instance.
$obj = new ClassName($value);

<?php
// Basic class definition.
class User {
	public string $name;
	public int $age;

	public function __construct(string $name, int $age) {
		$this->name = $name;
		$this->age = $age;
	}

	public function greet(): string {
		return "Hello, I'm {$this->name}. I'm {$this->age} years old.";
	}
}

$user = new User("Taro", 25);
echo $user->greet(); // Outputs: "Hello, I'm Taro. I'm 25 years old."
echo $user->name;    // Outputs: "Taro"

// Using constructor promotion introduced in PHP 8.0 for more concise code.
class Product {
	public function __construct(
		public readonly string $name,
		public readonly int $price,
		public readonly int $stock = 0
	) {}

	public function format_price(): string {
		return "¥" . number_format($this->price);
	}
}

$product = new Product("PHP Textbook", 2980, 50);
echo $product->format_price(); // Outputs: "¥2,980"
echo $product->name;           // Outputs: "PHP Textbook"

// Example using a destructor.
class FileHandler {
	private $handle;

	public function __construct(string $path) {
		$this->handle = fopen($path, 'r');
		echo "File opened.\n";
	}

	public function __destruct() {
		if ($this->handle) {
			fclose($this->handle);
		}
		echo "File closed.\n";
	}
}

$file = new FileHandler("/tmp/test.txt");
unset($file); // The destructor runs here.

// Example of method chaining.
class QueryBuilder {
	private string $table = '';
	private string $where = '';

	public function from(string $table): self {
		$this->table = $table;
		return $this; // Returning $this enables chaining.
	}

	public function where(string $condition): self {
		$this->where = $condition;
		return $this;
	}

	public function build(): string {
		return "SELECT * FROM {$this->table} WHERE {$this->where}";
	}
}

$sql = (new QueryBuilder())->from('users')->where('age > 20')->build();
echo $sql; // Outputs: "SELECT * FROM users WHERE age > 20"

A class is the fundamental unit of object-oriented programming — a blueprint that groups related data and the methods that operate on it. When you create an instance with the new keyword, __construct() runs automatically to set up initial values.

Constructor promotion, introduced in PHP 8.0, lets you declare, type-hint, and assign a property all in one line. Adding the readonly modifier prevents the property from being changed after initialization, which is useful for creating immutable objects.

For access modifiers, see public / private / protected. For inheritance, see extends / implements. For traits, see trait.