PHP Manual: constant

Description

function constant(string $name): mixed

Return the value of the constant indicated by name.

constant() is useful if you need to retrieve the value of a constant, but do not know its name. I.e. it is stored in a variable or returned by a function.

This function works also with class constants and enum cases.

Parameters

name: The constant name.

Note:
constant() takes the name as an ordinary string and does not resolve it against the current namespace: the name is looked up in the global namespace, so an unqualified name may silently return a different constant from the one the same bare identifier would refer to. To retrieve the value of a constant declared in a namespace, the namespace must be part of the name, as in constant('My\Namespace\MYCONST') or constant(__NAMESPACE__ . '\MYCONST'). Because the name is a plain string, aliases introduced by use statements and the namespace\ prefix are not applied to it. In a double-quoted string the namespace separators must be escaped, as in "My\\Namespace\\MYCONST". The namespace part of the name is case-insensitive, the constant name is not.

Return Values

Returns the value of the constant.

Errors/Exceptions

If the constant is not defined, an Error exception is thrown. Prior to PHP 8.0.0, an E_WARNING level error was generated in that case.

Changelog

Version	Description
8.0.0	If the constant is not defined, constant() now throws an Error exception; previously an `E_WARNING` was generated, and `null` was returned.

Examples

Example #1 Using constant() with Constants

<?php

define("MAXSIZE", 100);

echo MAXSIZE;
echo constant("MAXSIZE"); // same thing as the previous line


interface bar {
    const test = 'foobar!';
}

class foo {
    const test = 'foobar!';
}

$const = 'test';

var_dump(constant('bar::'. $const)); // string(7) "foobar!"
var_dump(constant('foo::'. $const)); // string(7) "foobar!"

?>

Example #2 Using constant() with Enum Cases (as of PHP 8.1.0)

<?php

enum Suit
{
    case Hearts;
    case Diamonds;
    case Clubs;
    case Spades;
}

$case = 'Hearts';

var_dump(constant('Suit::'. $case)); // enum(Suit::Hearts)

?>

constant