PHP Constructor

Class Constructors (__construct()) & Constructor Promotion

A constructor is a magic method (__construct()) that executes automatically when a new instance of a class is created. PHP 8 introduced Constructor Property Promotion, allowing properties to be declared and initialized directly inside the constructor signature.

Traditional vs PHP 8 Constructor Promotion

// Traditional Constructor
class User {
    public string $name;
    public function __construct(string $name) {
        $this->name = $name;
    }
}

// Modern PHP 8 Constructor Property Promotion!
class User {
    public function __construct(public string $name) {}
}

Constructor Workflow

flowchart LR
    A["new User('Maksudur', 28)"] --> B["__construct() Invoked"]
    B --> C["Promoted Properties Initialized Automatically"]
    C --> D["Instance Ready for Method Invocations"]

Practical Code Example

<?php
declare(strict_types=1);

class CustomerOrder
{
    // Modern PHP 8 Constructor Property Promotion
    public function __construct(
        public int $orderId,
        public string $customerName,
        public float $amount,
        public string $status = 'pending' // Default parameter
    ) {
        // Validation inside constructor
        if ($amount <= 0) {
            throw new InvalidArgumentException("Order amount must be greater than zero.");
        }
    }

    public function getOrderSummary(): string
    {
        return "Order #{$this->orderId} for {$this->customerName} - Total: ${$this->amount} [Status: {$this->status}]";
    }
}

// Instantiating object using promoted constructor
$order = new CustomerOrder(1001, "Maksudur", 249.50);
echo $order->getOrderSummary() . "
";

Best Practices

• Use PHP 8 Constructor Property Promotion: Eliminates boilerplate property declarations and assignment code.
• Validate Inputs Inside Constructors: Ensure objects cannot be instantiated in an invalid state by validating arguments during construction.
• Assign Default Parameter Values: Set default fallback parameters ($status = 'pending') for optional constructor arguments.

Self-Check Challenge

Refactor a class User to use PHP 8 Constructor Property Promotion to initialize public string $email and public int $id!