Constants

A constant is a name for a value that never changes during a script — a tax rate, a maximum upload size, a version number. Constants have no dollar sign, can't be reassigned, and are available everywhere. PHP also predefines hundreds of constants, including "magic" ones that describe where the code is.

Defining constants with const

Use the const keyword at the top level of a file (or inside a class — lesson 36). By convention, constant names are in UPPER_SNAKE_CASE:

PHP

<?php

const VAT_RATE = 0.2;
const APP_NAME = "Planner";
const WEEKDAYS = ["Mon", "Tue", "Wed", "Thu", "Fri"];

$net = 50;
echo APP_NAME, ": gross price ", $net * (1 + VAT_RATE), "\n";
echo count(WEEKDAYS), " working days, starting ", WEEKDAYS[0], "\n";

Output

Planner: gross price 60
5 working days, starting Mon

Constants can hold scalars, arrays, enum cases and (since PHP 8.1) objects created with new, and their value can be an expression built from other constants: const SECONDS_PER_DAY = 24 * 60 * 60;.

Constants can't change

PHP

<?php

const MAX_USERS = 100;
MAX_USERS = 200;

Output

Parse error: syntax error, unexpected token "=" in main.php on line 4

Defining the same constant twice also fails, with a warning, and the original value is kept.

define()

The older define() function creates a constant at run time. Unlike const, it can be called inside an if and the name can be computed. defined() checks whether a constant exists, and constant() reads one by name:

PHP

<?php

if (!defined('DEBUG')) {
    define('DEBUG', false);
}

$level = 'LOG_LEVEL_' . 'INFO';
define($level, 2);

var_dump(DEBUG);
echo constant('LOG_LEVEL_INFO'), "\n";

Output

bool(false)
2

Prefer const for fixed values; it's checked at compile time and reads more clearly.

Predefined constants

PHP provides constants describing the environment and limits:

PHP

<?php

echo PHP_MAJOR_VERSION, ".", PHP_MINOR_VERSION, "\n";
echo PHP_INT_MAX, "\n";
echo PHP_FLOAT_EPSILON, "\n";
echo PHP_OS_FAMILY === 'Windows' ? "Windows\n" : "Unix-like\n";
echo "Line one", PHP_EOL, "Line two", PHP_EOL;
echo M_PI, "\n";

Output

8.5
9223372036854775807
2.2204460492503E-16
Unix-like
Line one
Line two
3.1415926535898

PHP_EOL is the operating system's line ending — handy when writing text files. Many extensions add their own constants, such as JSON_PRETTY_PRINT, E_ALL or SORT_NUMERIC, which you pass as options to functions.

Magic constants

Magic constants start and end with two underscores, and their value depends on where they appear:

PHP

<?php

function whereAmI(): void
{
    echo "Function: ", __FUNCTION__, ", line ", __LINE__, "\n";
}

whereAmI();
echo "File: ", basename(__FILE__), "\n";
echo "Line: ", __LINE__, "\n";
echo "Directory exists: ", is_dir(__DIR__) ? "yes" : "no", "\n";

Output

Function: whereAmI, line 5
File: main.php
Line: 10
Directory exists: yes

Constant	Value
`__LINE__`	Current line number
`__FILE__`	Full path of the current file
`__DIR__`	Directory of the current file
`__FUNCTION__`, `__METHOD__`	Current function / class method name
`__CLASS__`, `__NAMESPACE__`	Current class / namespace

__DIR__ is especially useful for including files relative to the current one: require __DIR__ . '/config.php'; works no matter which directory the script was started from.

Constants or enums?

A group of related constants — order statuses, user roles, sizes — is better expressed as an enum (lesson 40), which the type system can check. Use plain constants for single configuration values.

Try it yourself

Define constants for a shop's free-shipping threshold and shipping fee, and use them to price a few orders.

Output

Order  12.50 + shipping 4.99 =  17.49
Order  49.99 + shipping 4.99 =  54.98
Order  50.00 + shipping 0.00 =  50.00
Order 120.00 + shipping 0.00 = 120.00

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