Constants and static

Class Constants and the static Keyword

A class constant (Class::NAME) may be private (PHP 7.1), final (8.1) and typed (8.3, Subsection 4.11.11). A static property holds one value per class; a static method has no $this. self:: names the current class, and PHP 8.3 added Class::{$name}.

Constants, a static counter and dynamic constant fetchPHP

<?php
class Product {
  public const string CURRENCY = 'USD';          // typed constant (PHP 8.3)
  final public const int MAX_QTY = 99;           // children cannot redefine it
  private const TAX_RATE = 0.08;
  public const PRICE_WITH_TAX = 1000 * (1 + self::TAX_RATE);   // constant expression
  private static int $created = 0;               // one value for the whole class
  public function __construct(public string $sku) { self::$created++; }
  public static function created(): int { return self::$created; }
}
new Product('BK-101'); new Product('BK-202');
$name = 'MAX_QTY';
echo Product::created(), ' ', Product::CURRENCY, ' ', Product::{$name}, ' ',
  Product::PRICE_WITH_TAX, "\n";
try { echo Product::TAX_RATE; } catch (Error $e) { echo $e->getMessage(), "\n"; }

Output

2 USD 99 1080
Cannot access private constant Product::TAX_RATE

Static state lasts one request under PHP-FPM, but in a long-running worker (Asynchronous PHP) it survives between requests and users; prefer injected objects (Subsection 4.9.15).

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Constants and static