Define vs Const in PHP: Key Differences and Usage

In PHP, define() is a function used to declare constants at runtime, while const is a language construct used to declare constants at compile time. const supports only scalar values and is limited to global or class scope, whereas define() can define constants dynamically and globally.

Quick Comparison

This table summarizes the main differences between define() and const in PHP.

Feature	define()	const
Declaration time	Runtime (during script execution)	Compile time (before script runs)
Syntax type	Function call	Language construct
Value types allowed	Any scalar value or expression	Only scalar values (int, float, string, bool)
Scope	Global only	Global or class constants
Namespace support	No	Yes
Usage in classes	No	Yes

Key Differences

define() is a function that defines constants at runtime, meaning you can use variables or expressions to set the constant's value. It only creates global constants and does not support namespaces or class constants.

On the other hand, const is a language construct evaluated at compile time, so its value must be a constant expression. It supports namespaces and can be used to declare constants inside classes, making it more versatile for object-oriented programming.

Additionally, const constants are slightly faster because they are resolved during compilation, while define() constants are resolved during execution. However, define() allows dynamic constant names and values, which const does not.

Code Comparison

Here is how you define a constant using define() in PHP:

php

<?php
// Using define() to create a constant
define('SITE_NAME', 'MyWebsite');
echo SITE_NAME;

Output

MyWebsite

const Equivalent

Here is the equivalent constant declaration using const:

php

<?php
// Using const to create a constant
const SITE_NAME = 'MyWebsite';
echo SITE_NAME;

Output

MyWebsite

When to Use Which

Choose const when you want to declare constants inside classes or namespaces, or when you need compile-time constants for better performance and clarity. Use define() when you need to create constants dynamically at runtime or when working in global scope without namespaces.

In modern PHP development, prefer const for class and global constants due to its better integration with namespaces and object-oriented features.

Key Takeaways

const is for compile-time, class, and namespace constants.

define() creates global constants at runtime and supports dynamic values.

const is faster and preferred in modern PHP code.

Use define() for dynamic or conditional constant definitions.

const supports only scalar values; define() is more flexible.