PHP Constants

PHP Constants & Magic Constants

Constants are identifiers for simple values that cannot be altered or undefined during script execution. Unlike variables, constants do not start with a dollar sign $, are globally accessible throughout script execution, and are defined using either const or define().

const vs define()

Magic Constants in PHP

PHP provides 8 magic constants that change dynamically based on where they are used:

• __LINE__: Current line number in file.
• __FILE__: Full path and filename of current file.
• __DIR__: Directory path of current file.
• __FUNCTION__: Current function name.
• __CLASS__: Current class name including namespace.
• __METHOD__: Current class method name.
• __NAMESPACE__: Current namespace name.

flowchart TD
    A["Constants Definition"] --> B["Global / Scope Constants"]
    A --> C["Magic Constants"]
    B --> B1["const API_KEY = 'xyz'"]
    B --> B2["define('APP_ENV', 'production')"]
    C --> C1["__DIR__ (File Directory Path)"]
    C --> C2["__FILE__ (Absolute File Path)"]
    C --> C3["__CLASS__ / __METHOD__"]

Practical Code Example

<?php
declare(strict_types=1);

namespace App\Config;

// Compile-time Constant
const MAX_UPLOAD_LIMIT_MB = 25;

// Runtime Constant Definition
define('APP_START_TIMESTAMP', microtime(true));

class Environment
{
    public const STAGING_URL = "https://staging.kodersolution.com";

    public function debugInfo(): array
    {
        return [
            'file' => __FILE__,
            'directory' => __DIR__,
            'class' => __CLASS__,
            'method' => __METHOD__,
            'line' => __LINE__,
        ];
    }
}

$env = new Environment();
echo "Max Upload Limit: " . MAX_UPLOAD_LIMIT_MB . " MB
";
echo "Staging URL: " . Environment::STAGING_URL . "
";
echo "Current File Directory: " . __DIR__ . "
";

Best Practices

• Prefer const over define() for Hardcoded Configuration: const is cleaner, faster, and works natively inside classes and interfaces.
• Use Magic Constant __DIR__ for Autoloading & Includes: Use require_once __DIR__ . '/../vendor/autoload.php'; instead of fragile relative file paths.
• Use UPPERCASE_SNAKE_CASE Naming Convention: Keep constant names in uppercase letters separated by underscores (DEFAULT_TIMEZONE).

Self-Check Challenge

Write a PHP class DatabaseConfig with a public const DB_PORT = 3306; and a method getConfigFile() that returns __DIR__ . '/config.json'!