PHP Constants

Constants are like variables that never change. Once defined, their value remains the same throughout your program. They're perfect for configuration values, API keys, and other fixed data!

What Are Constants?

A constant is an identifier for a simple value that cannot be changed during script execution. Unlike variables:

• Constants do NOT start with $
• Constants are automatically global
• Constants cannot be redefined or undefined
• Constants can only hold scalar values (not arrays or objects in PHP < 5.6)

Click Run to execute your code

Defining Constants: define()

The define() function creates a constant at runtime:

<?php
// Syntax: define(name, value, case_insensitive)
define("SITE_NAME", "8gwifi.org");
define("MAX_USERS", 100);
define("PI", 3.14159);
define("DEBUG_MODE", true);

echo SITE_NAME;  // Output: 8gwifi.org
echo MAX_USERS;  // Output: 100
?>

Case-Insensitive Constants (Deprecated)

<?php
// Third parameter makes it case-insensitive (deprecated in PHP 7.3)
define("GREETING", "Hello", true);

echo GREETING;  // Works
echo greeting;  // Also works (but deprecated!)
?>

Defining Constants: const Keyword

The const keyword defines constants at compile time (PHP 5.3+):

<?php
const APP_VERSION = "1.0.0";
const MAX_LOGIN_ATTEMPTS = 3;
const ENABLE_CACHE = true;

echo APP_VERSION;  // Output: 1.0.0
?>

define() vs const

<?php
// define() can be conditional
if ($production) {
    define("DEBUG", false);
} else {
    define("DEBUG", true);
}

// const cannot be conditional
const DEBUG = false;  // ✅ Works at top level
if ($production) {
    const DEBUG = false;  // ❌ Syntax error!
}
?>

Array Constants (PHP 5.6+)

Since PHP 5.6, constants can hold arrays:

<?php
const COLORS = ["red", "green", "blue"];
define("SIZES", ["small", "medium", "large"]);

echo COLORS[0];  // Output: red
echo SIZES[1];   // Output: medium
?>

Checking if Constant Exists

Use defined() to check if a constant is defined:

<?php
if (defined("DEBUG_MODE")) {
    echo "Debug mode is defined";
}

// Get constant value safely
$debug = defined("DEBUG_MODE") ? DEBUG_MODE : false;
?>

Magic Constants

PHP provides special "magic constants" that change depending on where they're used:

<?php
echo "Current file: " . __FILE__;
echo "Current directory: " . __DIR__;
echo "Current line: " . __LINE__;

function myFunction() {
    echo "Function: " . __FUNCTION__;
}

class MyClass {
    public function myMethod() {
        echo "Class: " . __CLASS__;
        echo "Method: " . __METHOD__;
    }
}
?>

Predefined Constants

PHP has many built-in constants:

<?php
echo PHP_VERSION;  // Current PHP version
echo PHP_OS;       // Operating system
echo PHP_INT_MAX;  // Maximum integer value
?>

Class Constants

Constants can be defined inside classes:

<?php
class Database {
    const HOST = "localhost";
    const PORT = 3306;
    const NAME = "mydb";
    
    public function connect() {
        echo "Connecting to " . self::HOST;
    }
}

echo Database::HOST;  // Access from outside
?>

Best Practices

Common Mistakes

<?php
define("MAX_SIZE", 100);

echo $MAX_SIZE;  // ❌ Undefined variable
echo MAX_SIZE;   // ✅ Correct
?>

<?php
define("VERSION", "1.0");
define("VERSION", "2.0");  // ❌ Warning: already defined

VERSION = "2.0";  // ❌ Parse error
?>

<?php
if ($debug) {
    const DEBUG = true;  // ❌ Syntax error!
}

// Use define() instead
if ($debug) {
    define("DEBUG", true);  // ✅ Correct
}
?>

Exercise: Configuration Constants

<?php
// Application constants
const APP_NAME = "MyApp";
const APP_VERSION = "1.0.0";
const ENVIRONMENT = "development";

// Database constants
define("DB_HOST", "localhost");
define("DB_NAME", "myapp_db");
define("DB_USER", "root");
define("DB_PASS", "");

// Display configuration
echo "Application: " . APP_NAME . " v" . APP_VERSION . "\n";
echo "Environment: " . ENVIRONMENT . "\n";
echo "Config file: " . __FILE__ . "\n";

// Safe constant access
if (defined("DB_HOST")) {
    echo "Database: " . DB_HOST . "/" . DB_NAME;
}
?>

Summary

What's Next?

Congratulations! You've completed Module 2 and mastered variables, data types, type juggling, strings, and constants. In the next module, we'll dive into Operators - the symbols that let you perform calculations, comparisons, and logical operations!