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Syntax

Constants can be defined using the const keyword, or by using the define()-function. While define() allows a constant to be defined to an arbitrary expression, the const keyword has restrictions as outlined in the next paragraph. Once a constant is defined, it can never be changed or undefined.

When using the const keyword, only scalar (bool, int, float and string) expressions and constant arrays containing only scalar expressions are accepted. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.

The value of a constant is accessed simply by specifying its name. Unlike variables, a constant is not prepended with a $. It is also possible to use the constant() function to read a constant's value if the constant's name is obtainned dynamically. Use get_defined_constants() to get a list of all defined constants.

Note: Constants and (global) variables are in a different namespace. This implies that for example true and $TRUE are generally different.

If an undefined constant is used an Error is thrown. Prior to PHP 8.0.0, undefined constants would be interpreted as a bare word string, i.e. (CONSTANT vs "CONSTANT"). This fallback is deprecated as of PHP 7.2.0, and an error of level E_WARNING is issued when it happens. Prior to PHP 7.2.0, an error of level E_NOTICE has been issued instead. See also the manual entry on why $foo[bar] is wrong (unless bar is a constant). This does not apply to (fully) qualified constants, which will always raise a Error if undefined.

Note: To check if a constant is set, use the defined() function.

These are the differences between constants and variables:

• Constants do not have a dollar sign ($) before them;
• Constants may be defined and accessed anywhere without regard to variable scoping rules;
• Constants may not be redefined or undefined once they have been set; and
• Constants may only evaluate to scalar values or arrays.

<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // Emits an Error: Undefined constant "Constant"
               // Prior to PHP 8.0.0, outputs "Constant" and issues a warning.
?>

<?php
// Simple scalar value
const CONSTANT = 'Hello World';

echo CONSTANT;

// Scalar expression
const ANOTHER_CONST = CONSTANT.'; Goodbye World';
echo ANOTHER_CONST;

const ANIMALS = array('dog', 'cat', 'bird');
echo ANIMALS[1]; // outputs "cat"

// Constant arrays
define('ANIMALS', array(
    'dog',
    'cat',
    'bird'
));
echo ANIMALS[1]; // outputs "cat"
?>

Note:

As opposed to defining constants using define(), constants defined using the const keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops, if statements or try/ catch blocks.