PHP Constants: define() vs const and Magic Constants

A constant is a name for a value that never changes while the script runs. Declare it with const MAX_USERS = 100; or define('MAX_USERS', 100);, then use the bare name, without a $.

<?php
const SITE_NAME = "Coffee Corner";
define("MAX_ITEMS", 5);

echo SITE_NAME, "\n";
echo "You can order up to " . MAX_ITEMS . " items\n";

$price = 3.5;
const TAX_RATE = 0.2;
echo "Total: ", $price * (1 + TAX_RATE), "\n";

By convention constant names are UPPER_SNAKE_CASE. They are global: a constant declared at the top of a file is visible inside every function without global.

define() vs const

Both create a constant, but they work at different times. const is resolved when PHP compiles the file, define() runs like any other function call.

<?php
// const accepts expressions made of literals and other constants
const SECONDS_PER_DAY = 24 * 60 * 60;
const GREETING = "Hello, " . "world";

// define() can run conditionally and take a computed name
$env = "dev";
if ($env === "dev") {
    define("DEBUG", true);
}
foreach (["RED" => "#f00", "GREEN" => "#0f0"] as $name => $hex) {
    define("COLOR_" . $name, $hex);
}

echo SECONDS_PER_DAY, "\n";
echo GREETING, "\n";
var_dump(DEBUG);
echo COLOR_GREEN, "\n";

Use const by default, because it is shorter and checked at compile time. Use define() when the constant depends on a condition or a computed name.

Constants cannot be changed

Assigning to a constant is a parse error:

<?php
const LIMIT = 10;
LIMIT = 20;

PHP Parse error:  syntax error, unexpected token "=" in /home/index.php on line 3

Defining the same constant twice is not an error, but PHP prints a warning and keeps the first value:

<?php
define("LIMIT", 10);
define("LIMIT", 20);
echo LIMIT; // 10

Warning: Constant LIMIT already defined in /home/index.php on line 3

When a value might already be set (for example in a config file loaded more than once), check with defined() first:

<?php
if (!defined("APP_ENV")) {
    define("APP_ENV", "production");
}
if (!defined("APP_ENV")) {
    define("APP_ENV", "testing");   // skipped
}
echo APP_ENV, "\n";

var_dump(defined("APP_ENV"));
var_dump(defined("NOT_THERE"));

// Read a constant whose name is in a string
$name = "APP_ENV";
echo constant($name), "\n";

Constant arrays

A constant can hold an array. You can read it with NAME[key], loop over it, and pass it around, but not modify it.

<?php
const WEEKDAYS = ["Mon", "Tue", "Wed", "Thu", "Fri"];
define("HTTP_CODES", [200 => "OK", 404 => "Not Found", 500 => "Server Error"]);

echo WEEKDAYS[0], "\n";
echo count(WEEKDAYS), " weekdays\n";
echo HTTP_CODES[404], "\n";

foreach (HTTP_CODES as $code => $text) {
    echo "$code $text\n";
}

Class constants

Inside a class, const declares a constant that belongs to the class. Read it with ClassName::NAME from outside and self::NAME or static::NAME from inside. Class constants can have visibility, can be final, and since PHP 8.3 can declare a type.

<?php
class Order
{
    public const STATUS_NEW = "new";
    public const STATUS_PAID = "paid";
    private const MAX_LINES = 50;
    final public const string CURRENCY = "EUR";   // typed, PHP 8.3

    public function __construct(public string $status = self::STATUS_NEW) {}

    public function canAddLine(int $lines): bool
    {
        return $lines < self::MAX_LINES;
    }
}

$order = new Order();
echo $order->status, "\n";
echo Order::STATUS_PAID, "\n";
echo Order::CURRENCY, "\n";
var_dump($order->canAddLine(10));

// Dynamic access by name
$which = "STATUS_PAID";
echo constant("Order::$which"), "\n";

A group of related class constants like STATUS_NEW and STATUS_PAID is often better written as an enum, which also stops invalid values from being passed around.

Magic constants: LINE, FILE, DIR and more

Magic constants change value depending on where they appear. They are filled in at compile time.

<?php
namespace App\Billing;

class Invoice
{
    public function describe(): string
    {
        return "class: " . __CLASS__ . "\n"
            . "method: " . __METHOD__ . "\n"
            . "function: " . __FUNCTION__ . "\n";
    }
}

echo (new Invoice())->describe();
echo "namespace: ", __NAMESPACE__, "\n";
echo "line: ", __LINE__, "\n";
echo "file: ", basename(__FILE__), "\n";
echo "dir is a directory: ", var_export(is_dir(__DIR__), true), "\n";

The most used one is __DIR__, the directory of the current file. Building include paths from it makes them work no matter which directory the script was started from: require __DIR__ . '/config.php';. See include and require for the full pattern.

Built-in constants worth knowing

PHP defines many constants of its own. These come up all the time:

<?php
echo "PHP_VERSION:   ", PHP_VERSION, "\n";
echo "PHP_INT_MAX:   ", PHP_INT_MAX, "\n";
echo "PHP_INT_SIZE:  ", PHP_INT_SIZE, " bytes\n";
echo "PHP_FLOAT_EPSILON: ", PHP_FLOAT_EPSILON, "\n";
echo "M_PI:          ", M_PI, "\n";
echo "PHP_OS_FAMILY: ", PHP_OS_FAMILY, "\n";
echo "E_ALL:         ", E_ALL, "\n";
echo "PHP_EOL is a newline: ", var_export(PHP_EOL === "\n", true), PHP_EOL;

PHP_EOL is the line ending of the system PHP runs on ("\n" on Linux and macOS, "\r\n" on Windows). It is meant for text written to files and the terminal; in HTML output a newline does not show, so use <br> there.