PHP Constants

What are Constants?

Constants are like variables, but their value cannot be changed once defined. They’re useful for values that should remain fixed throughout your application, like configuration settings.

define('SITE_NAME', 'My Website');
echo SITE_NAME;  // My Website

// SITE_NAME = 'Other';  // Error: cannot reassign a constant

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Defining Constants

Using define()

Works anywhere in your code, including inside functions and conditionals.

define('MAX_USERS', 1000);
define('TAX_RATE', 0.085);
define('DEBUG_MODE', true);
define('ALLOWED_TYPES', ['jpg', 'png', 'gif']);

Using const

Must be declared at the top level of a script or inside a class. Slightly faster performance.

const MAX_USERS = 1000;
const TAX_RATE = 0.085;
const DEBUG_MODE = true;
const ALLOWED_TYPES = ['jpg', 'png', 'gif'];

Key Differences

Feature	define()	const
Location	Anywhere (functions, conditionals)	Top level or class only
Name	Can use variables	Must be literal string
Speed	Slightly slower	Slightly faster

---

Checking and Accessing Constants

// Check if defined
if (defined('MAX_USERS')) {
    echo MAX_USERS;
}

// Access array constant elements
echo ALLOWED_TYPES[0];  // jpg

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Class Constants

Constants can be defined inside classes and accessed with ::.

class AppConfig {
    const VERSION = '2.1.0';
    const MAX_FILE_SIZE = 5242880;
    const SUPPORTED_FORMATS = ['json', 'xml', 'csv'];
}

echo AppConfig::VERSION;           // 2.1.0
echo AppConfig::SUPPORTED_FORMATS[0];  // json

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Magic Constants

Magic constants are predefined by PHP and provide information about the current script. They start and end with double underscores and their values change based on where they’re used.

__LINE__

Current line number.

echo __LINE__;  // 5

__FILE__

Full path and filename.

echo __FILE__;           // /var/www/html/index.php
echo basename(__FILE__); // index.php

__DIR__

Directory of the current file.

echo __DIR__;  // /var/www/html
include __DIR__ . '/config.php';

__FUNCTION__

Current function name.

function process() {
    echo __FUNCTION__;  // process
}

__CLASS__

Current class name.

class User {
    public function getName() {
        echo __CLASS__;  // User
    }
}

__METHOD__

Current class and method name.

class User {
    public function save() {
        echo __METHOD__;  // User::save
    }
}

__NAMESPACE__

Current namespace.

namespace App\Controllers;

echo __NAMESPACE__;  // App\Controllers

__TRAIT__

Current trait name.

trait Loggable {
    public function log() {
        echo __TRAIT__;  // Loggable
    }
}

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Practical Example

Magic constants are useful for debugging and logging:

function logError($message) {
    $entry = sprintf(
        "[%s] %s:%d - %s",
        date('Y-m-d H:i:s'),
        __FILE__,
        __LINE__,
        $message
    );
    error_log($entry);
}