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 obtained 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.

Example #1 Defining Constants

<?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.
?>

Example #2 Defining Constants using the const keyword

<?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.

See Also

Here you can write a comment


Please enter at least 10 characters.
Loading... Please wait.
* Pflichtangabe
There are no comments available yet.

PHP cURL Tutorial: Using cURL to Make HTTP Requests

cURL is a powerful PHP extension that allows you to communicate with different servers using various protocols, including HTTP, HTTPS, FTP, and more. ...

TheMax

Autor : TheMax
Category: PHP-Tutorials

Midjourney Tutorial - Instructions for beginners

There is an informative video about Midjourney, the tool for creating digital images using artificial intelligence, entitled "Midjourney tutorial in German - instructions for beginners" ...

Mike94

Autor : Mike94
Category: KI Tutorials

Basics of views in MySQL

Views in a MySQL database offer the option of creating a virtual table based on the result of an SQL query. This virtual table can be queried like a normal table without changing the underlying data. ...

admin

Autor : admin
Category: mySQL-Tutorials

Publish a tutorial

Share your knowledge with other developers worldwide

Share your knowledge with other developers worldwide

You are a professional in your field and want to share your knowledge, then sign up now and share it with our PHP community

learn more

Publish a tutorial