PHP Constants

Class Constants (const, Visibility Modifiers, self::)

Class constants are immutable values allocated per class definition rather than per object instance. Starting in PHP 7.1, class constants support access visibility modifiers (public const, protected const, private const).

Class Constant Features

• Declared with const keyword (Without $ sign).
• Referenced inside class via self::CONSTANT_NAME.
• Referenced outside class via ClassName::CONSTANT_NAME.
• Value cannot be mutated after definition.

flowchart LR
    A["ClassName::API_VERSION"] --> B["Class Constant Evaluation"]
    C["self::MAX_RETRY_LIMIT"] --> B
    B --> D["Shared Immutable Value Across All Objects"]

Practical Code Example

<?php
declare(strict_types=1);

class PaymentGateway
{
    // Public Class Constant (Accessible globally)
    public const CURRENCY_USD = "USD";
    public const CURRENCY_EUR = "EUR";

    // Protected Class Constant (Accessible only in class & subclasses)
    protected const API_VERSION = "v2.5";

    // Private Class Constant (Encapsulated internally)
    private const MAX_RETRIES = 3;

    public function processTransaction(float $amount, string $currency): string
    {
        if (!in_array($currency, [self::CURRENCY_USD, self::CURRENCY_EUR], true)) {
            throw new InvalidArgumentException("Unsupported Currency");
        }

        return "Processing $" . $amount . " " . $currency . " via API " . self::API_VERSION . " (Max Retries: " . self::MAX_RETRIES . ")";
    }
}

$gateway = new PaymentGateway();
echo "Supported Currency: " . PaymentGateway::CURRENCY_USD . "
";
echo $gateway->processTransaction(100.0, PaymentGateway::CURRENCY_USD) . "
";

Best Practices

• Always Explicitly State Visibility Modifiers: Declare constants as public const, protected const, or private const.
• Use UPPERCASE_SNAKE_CASE Formatting: Follow standard constant naming (STATUS_ACTIVE, DEFAULT_PORT).
• Use self:: for Internal References: Reference class constants internally via self::CONST_NAME rather than hardcoding values.

Self-Check Challenge

Write a class Status containing public const DRAFT = 'draft'; and access it outside the class using Status::DRAFT!