PHP Constants

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Constants are like variables, but they cannot be changed once they are defined.

How to define a constant?

• In PHP, you can define constants using the define() function or the const keyword.
• A constant can be defined with various data types.

Important notes

• Array constants are supported in define() since PHP 7.0 and in const since PHP 5.6,
• Constants are always case-sensitive,
• Once defined, constants cannot be changed or undefined,
• Resource type constants are not allowed,
• Object constants are not directly supported (except for simple objects in some cases).

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1. Using the define() function

// String constants
define('SITE_NAME', 'My Website');
define('DATABASE_HOST', 'localhost');

// Integer constants
define('MAX_USERS', 1000);
define('DEFAULT_PORT', 3306);

// Float constants
define('PI', 3.14159);
define('TAX_RATE', 0.085);

// Boolean constants
define('DEBUG_MODE', true);
define('MAINTENANCE_MODE', false);

// Array constants (PHP 7.0+)
define('ALLOWED_EXTENSIONS', ['jpg', 'png', 'gif', 'pdf']);
define('DATABASE_CONFIG', [
    'host' => 'localhost',
    'port' => 3306,
    'name' => 'myapp'
]);

// Null constant
define('DEFAULT_AVATAR', null);

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2. Using the const keyword (PHP 5.3+)

// String constants
const SITE_NAME = 'My Website';
const DATABASE_HOST = 'localhost';

// Integer constants
const MAX_USERS = 1000;
const DEFAULT_PORT = 3306;

// Float constants
const PI = 3.14159;
const TAX_RATE = 0.085;

// Boolean constants
const DEBUG_MODE = true;
const MAINTENANCE_MODE = false;

// Array constants (PHP 5.6+)
const ALLOWED_EXTENSIONS = ['jpg', 'png', 'gif', 'pdf'];
const DATABASE_CONFIG = [
    'host' => 'localhost',
    'port' => 3306,
    'name' => 'myapp'
];

// Null constant
const DEFAULT_AVATAR = null;

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Key differences

1. define() function:

   • Can be used anywhere in your code (including inside functions, conditionals, etc.),
   • Can use variables or expressions for the constant name,
   • Works with all PHP versions,
   • Array constants supported since PHP 7.0.

2. const keyword:

   • Must be declared at the top level of the script (not inside functions or classes, except for class constants),
   • The constant name must be a literal string,
   • Only available in PHP 5.3 and later,
   • Slightly faster performance,
   • Array constants supported since PHP 5.6.

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Checking if a constant is defined

if (defined("CONSTANT_NAME")) {
    echo "CONSTANT_NAME is defined";
} else {
    echo "CONSTANT_NAME is not defined";
}

Accessing a constant

echo CONSTANT_NAME;

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Example usage:

// String
echo SITE_NAME;  // Outputs: My Website

// Integer
echo MAX_USERS;  // Outputs: 1000

// Float
echo PI;         // Outputs: 3.14159

// Boolean
var_dump(DEBUG_MODE);  // Outputs: bool(true)

// Array
print_r(ALLOWED_EXTENSIONS);
// Outputs: Array ( [0] => jpg [1] => png [2] => gif [3] => pdf )

// Accessing array elements
echo ALLOWED_EXTENSIONS[0];  // Outputs: jpg
echo DATABASE_CONFIG['host']; // Outputs: localhost

// Null
var_dump(DEFAULT_AVATAR);    // Outputs: NULL

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Class constants with different data types:

AppConfig.php

class AppConfig {
    const VERSION = '2.1.0';              // String
    const MAX_FILE_SIZE = 5242880;         // Integer (5MB in bytes)
    const UPLOAD_RATE_LIMIT = 10.5;        // Float
    const CACHE_ENABLED = true;            // Boolean
    const SUPPORTED_FORMATS = ['json', 'xml', 'csv']; // Array
    const DEFAULT_THEME = null;            // Null
}

echo AppConfig::VERSION;                   // Outputs: 2.1.0
echo AppConfig::MAX_FILE_SIZE;             // Outputs: 5242880
var_dump(AppConfig::CACHE_ENABLED);        // Outputs: bool(true)
print_r(AppConfig::SUPPORTED_FORMATS);     // Outputs array

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Magic Constants

Magic constants are predefined constants in PHP that provide contextual information about the script and its execution environment. They are automatically defined by PHP and change their value depending on where they are used in the code.

Magic constants start and end with double underscores (__) and are case-insensitive. Their values are resolved at compile time, not runtime.

Available Magic Constants

__LINE__

Returns the current line number of the file.

echo "This is line number: " . __LINE__;  // Outputs: This is line number: 2
echo "This is line number: " . __LINE__;  // Outputs: This is line number: 3

__FILE__

Returns the full path and filename of the current file (with symlinks resolved).

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

__DIR__

Returns the directory of the current file (equivalent to dirname(__FILE__)).

echo __DIR__;  // Outputs: /var/www/html
include __DIR__ . '/config.php';  // Include file from same directory

__FUNCTION__

Returns the name of the current function (case-sensitive as declared).

function getUserData() {
    echo "Currently in function: " . __FUNCTION__;  // Outputs: getUserData
}

getUserData();

__CLASS__

Returns the name of the current class (case-sensitive as declared).

class UserManager {
    public function showClassName() {
        echo "Current class: " . __CLASS__;  // Outputs: UserManager
    }
}

$user = new UserManager();
$user->showClassName();

__METHOD__

Returns the class method name (includes class name and method name).

class DatabaseConnection {
    public function connect() {
        echo "Current method: " . __METHOD__;  // Outputs: DatabaseConnection::connect
    }

    private static function getConfig() {
        echo "Current method: " . __METHOD__;  // Outputs: DatabaseConnection::getConfig
    }
}

$db = new DatabaseConnection();
$db->connect();

__NAMESPACE__

Returns the name of the current namespace.

namespace App\Controllers;

class HomeController {
    public function index() {
        echo "Current namespace: " . __NAMESPACE__;  // Outputs: App\Controllers
        echo "Full class name: " . __NAMESPACE__ . '\\' . __CLASS__;  // App\Controllers\HomeController
    }
}

__TRAIT__

Returns the trait name (case-sensitive as declared).

trait Loggable {
    public function log($message) {
        echo "Logging from trait: " . __TRAIT__ . " - " . $message;  // Outputs: Loggable
    }
}

class User {
    use Loggable;
}

$user = new User();
$user->log("User created");

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Practical Examples

Error Logging with Context

function logError($message) {
    $logEntry = sprintf(
        "[%s] Error in %s on line %d: %s\n",
        date('Y-m-d H:i:s'),
        __FILE__,
        __LINE__,
        $message
    );
    error_log($logEntry);
}

// Usage
logError("Database connection failed");

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Dynamic File Inclusion

// Include configuration file from same directory
require_once __DIR__ . '/config.php';

// Include class file based on current namespace
$classFile = __DIR__ . '/' . str_replace('\\', '/', __NAMESPACE__) . '/' . __CLASS__ . '.php';

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Debug Information

class DebugHelper {
    public static function getDebugInfo() {
        return [
            'file' => __FILE__,
            'line' => __LINE__,
            'class' => __CLASS__,
            'method' => __METHOD__,
            'namespace' => __NAMESPACE__,
            'directory' => __DIR__
        ];
    }
}

// Usage
print_r(DebugHelper::getDebugInfo());

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Automatic Class Loading

function autoloader($className) {
    $file = __DIR__ . '/classes/' . str_replace('\\', '/', $className) . '.php';
    if (file_exists($file)) {
        require_once $file;
        echo "Loaded class from: " . $file . " (detected from " . __FILE__ . ")\n";
    }
}

spl_autoload_register('autoloader');

Usage Tips

• Magic constants are resolved at compile time, so they reflect where they appear in the source code
• Use __DIR__ instead of dirname(__FILE__) for better readability (PHP 5.3+)
• Magic constants are useful for debugging, logging, and dynamic file inclusion
• They help make your code more portable and maintainable

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