Constants (define vs const)

Also Known As

TL;DR

Explanation

PHP has two ways to define constants: const NAME = value (compile-time, usable in class bodies, interfaces, and at the top level, but not inside functions or conditionals) and define('NAME', value) (runtime, supports variable names, works anywhere). Class constants (const) support visibility modifiers since PHP 7.1 and typed constants since PHP 8.3. Use const for class-level constants and top-level application configuration; use define() when the constant name or value must be computed at runtime. Constants are globally accessible by name and cannot be redefined — prefer class constants or enums for namespaced, typed constant groups.

Common Misconception

Why It Matters

Common Mistakes

• Using define() inside functions or conditionally — prefer const at the top level for clarity and performance.
• Class constants not declared final in PHP 8.1 — subclasses can override them, violating the 'constant' contract.
• Using magic numbers instead of named constants — the meaning is lost and updates require searching the whole codebase.
• Naming constants with camelCase instead of SCREAMING_SNAKE_CASE — breaks PHP convention and reduces readability.

Code Examples

// Magic number — meaning unknown:
if ($status === 3) { /* What is 3? */ }

// Named constant:
const ORDER_STATUS_SHIPPED = 3;
if ($status === ORDER_STATUS_SHIPPED) { /* Clear */ }

// Non-final class constant — overrideable:
class Config {
    const TIMEOUT = 30; // Should be: final const TIMEOUT = 30; (PHP 8.1+)
}

// define() — runtime, works anywhere
define('MAX_UPLOAD', 10 * 1024 * 1024);
define('APP_ENV', getenv('APP_ENV') ?: 'production');

// const — compile-time, preferred in classes/namespaces
class Order {
    const STATUS_PENDING   = 'pending';
    const STATUS_PAID      = 'paid';
    const STATUS_CANCELLED = 'cancelled';
}
echo Order::STATUS_PAID;

// PHP 8.3 — typed class constants
class Config {
    const string VERSION     = '2.4.1';
    const int    MAX_RETRIES = 3;
}

// Useful built-in constants:
// PHP_EOL, PHP_INT_MAX, PHP_INT_SIZE
// DIRECTORY_SEPARATOR, PATH_SEPARATOR

References

• ↗ https://www.php.net/manual/en/language.constants.php

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