? PHP Constant Variables

? Quick Overview

PHP constants store fixed values that cannot be changed once defined. They are ideal for configuration values that remain the same throughout script execution.

? Key Concepts

Constants are defined using define()
No $ symbol is used
Constants are globally accessible

? Syntax & Theory

? View Code Example

// Basic syntax for defining a constant in PHP
define(name, value, case_insensitive);

? Code Example

? View Code Example

// Defining and using a constant
<?php
define("SITE_NAME", "MyWebsite");
echo SITE_NAME;
?>

? Output

MyWebsite

? Case-Insensitive Constant

? View Code Example

// Example of a case-insensitive constant (deprecated)
<?php
define("GREETING", "Welcome!", true);
echo greeting;
?>

⚡ Interactive Lab: Constant Lockdown

Simulate how PHP handles constants. Try defining a constant and then try to change its value.

1. Define Constant Name (e.g., API_KEY):

2. Set Initial Value:

3. Attempt to redefine to:

// Click run to see simulation results...

? Use Cases

Website configuration values
Application-wide settings
Fixed messages and limits

? Interactive Concept

Think of constants as locked labels in your application—once set, every part of your code can read them but never modify them.

✅ Tips & Best Practices

Use uppercase names for constants
Prefer constants for values that should not change
Avoid deprecated case-insensitive constants

? Try It Yourself

Create a PHP file named constant.php
Define a constant using define()
Echo it and try redefining it