PHP Constants

PHP Constants: Global Immutable Values and Magic Constants

A PHP constant is an identifier for a fixed value that cannot change or be undefined once your script defines it. Unlike variables, constants never start with a $ sign and are automatically readable inside every function and class in your script.

Defining Constants with const vs define()

PHP gives you two ways to declare constants. Choose const for clean top-level or class definitions, and define() when the value comes from a dynamic condition or runtime expression.

<?php
declare(strict_types=1);

const APP_NAME = "KoderSolution";
const SUPPORTED_LOCALES = ["en", "bn", "es"];

if (!defined('MAX_UPLOAD_MB')) {
    define('MAX_UPLOAD_MB', 10);
}

echo APP_NAME . " allows uploads up to " . MAX_UPLOAD_MB . " MB." . PHP_EOL;

Useful Built-In and Magic Constants

PHP provides predefined constants like PHP_VERSION and PHP_EOL, plus magic constants whose values change depending on where they appear in your file.

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

function logAction(): void
{
    echo "Inside function: " . __FUNCTION__ . PHP_EOL;
}

logAction();

Using __DIR__ when building file paths makes require statements reliable no matter which working directory started the script.

Common Mistakes to Avoid

• Prefixing a constant with $: Writing $APP_NAME looks for a variable instead of your constant.
• Redefining an existing constant: Calling define() twice with the same name throws a warning. Check defined('NAME') first if unsure.
• Putting const inside an if block: Placing const DEBUG = true; inside a conditional causes a ParseError. Use define() for conditional constants.

Frequently Asked Questions

What naming convention should I use for PHP constants?

Use uppercase letters with underscores (SCREAMING_SNAKE_CASE), such as API_TIMEOUT_SECONDS, so constants stand out clearly from variables.

Are PHP constants accessible inside functions automatically?

Yes. Top-level constants are globally scoped everywhere in your script without needing the global keyword.

Can a constant hold an array in PHP?

Yes. Since PHP 7, both const and define() support array constants, such as const ROLES = ['admin', 'member'];.