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PHP Constants

A constant is like a name written in permanent marker: once you decide what it stands for, it can never be changed. It is perfect for things that should stay the same, like the name of your website.

In this page:

  1. Defining Constants with define()
  2. The const Keyword
  3. Constant Scope
  4. Magic Constants
  5. Constant Best Practices
Syntax
php 
define("CONSTANT_NAME", value);
const CONSTANT_NAME = value;

echo CONSTANT_NAME;   // no $ sign

Defining Constants with define()

define(SITE_NAME, 'Cookies & Cursor') creates a constant at runtime.

Once set, a constant's value is locked for the rest of the script's execution — there's no reassignment operator for constants the way there is for variables, which is exactly the guarantee you want for values that must never change mid-script.

Example: Defining Constants with define()

php 
<?php
// Call `define('SITE_NAME', 'Cookies & Cursor')`
define('SITE_NAME', 'Cookies & Cursor');
// Print `SITE_NAME` to the output
echo SITE_NAME;
?>

The const Keyword

The const keyword defines a constant at compile time rather than runtime, which makes it slightly faster and is the standard way to declare constants inside a class.

Outside a class, define() and const both work, but const requires the value to be a fixed literal rather than the result of a function call.

Example: The const Keyword

php 
<?php
const MAX_USERS = 100;
// Print `MAX_USERS` to the output
echo MAX_USERS;
?>

Constant Scope

Unlike a variable, a constant is automatically accessible everywhere in the script the moment it's defined — inside every function and every included file — without needing the global keyword.

That global-by-default behavior is exactly why constants suit values like configuration settings or fixed limits.

Example: Constant Scope

php 
<?php
// Call `define('APP_VERSION', '1.0')`
define('APP_VERSION', '1.0');

// Define the function `showVersion` with no parameters
function showVersion() {
    // Print `APP_VERSION` to the output
    echo APP_VERSION;
}
// Call `showVersion()`
showVersion();
?>

Magic Constants

PHP predefines several 'magic constants' — like __LINE__ and __FILE__ — whose value changes depending on where in your code they appear.

__LINE__ always evaluates to the current line number, and __FILE__ to the full path of the file it's written in, which makes both genuinely useful for logging and debugging.

Example: Magic Constants

php 
<?php
// Print "Line: " . __LINE__ . "\n" to the output
echo "Line: " . __LINE__ . "\n";
// Print `"File: " . __FILE__` to the output
echo "File: " . __FILE__;
?>

Constant Best Practices

Writing constant names in ALL_CAPS isn't enforced by PHP, but it's a near-universal convention that lets a reader instantly tell MAX_RETRIES is a fixed constant rather than a variable that might change.

Grouping related constants together near the top of a file also makes configuration easy to find later.

Example: Constant Best Practices

php 
<?php
// Call `define('MAX_RETRIES', 3)`
define('MAX_RETRIES', 3);
// Call `define('MIN_RETRIES', 1)`
define('MIN_RETRIES', 1);
// Print `MAX_RETRIES` to the output
echo MAX_RETRIES;
?>
Related Topics

PHP Class Constants · PHP Magic Constants · PHP Variables · PHP Type Casting

{# common_mistakes/chapter_summary/browser_support: on Hindi pages the view already swaps in the hi_ translation fields (or blanks these out if untranslated), so this renders correctly for both languages without a lang_code check here. #}
Common Mistakes
  1. Writing $ before a constant name (echo $SITE_NAME;) or forgetting that constants are used bare, so PHP looks for a variable that does not exist.
  2. Trying to change a constant with SITE_NAME = New; or redefine it with define(), which fails or raises a warning because constants cannot change.
  3. Using const inside an if or a function, which is not allowed at top level scope only; use define() for conditional definitions.
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