Syntax

You can define a constant by using the define()-function or by using the const keyword outside a class definition as of PHP 5.3.0. 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 data (boolean, integer, float and string) can be contained in constants prior to PHP 5.6. From PHP 5.6 onwards, it is possible to define a constant as a scalar expression, and it is also possible to define an array constant. It is possible to define constants as a resource, but it should be avoided, as it can cause unexpected results.

You can get the value of a constant by simply specifying its name. Unlike with variables, you should not prepend a constant with a $. You can also use the function constant() to read a constant's value if you wish to obtain the constant's name 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 you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a string (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 (previously, an error of level E_NOTICE has been issued instead.) See also the manual entry on why $foo[bar] is wrong (unless you first define() bar as a constant). This does not apply to (fully) qualified constants, which will raise a fatal error if undefined. If you simply want 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;
Prior to PHP 5.3, Constants may only be defined using the define() function, not by simple assignment;
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. As of PHP 5.6 it is possible to define array constant using const keywords and as of PHP 7 array constants can also be defined using define() You may use arrays in constant scalar expressions (for example, const FOO = array(1,2,3)[0];), but the end result must be a value of allowed type.

Example #1 Defining Constants


<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.
?>

Example #2 Defining Constants using the const keyword


<?php
// Works as of PHP 5.3.0
const CONSTANT = 'Hello World';

echo CONSTANT;

// Works as of PHP 5.6.0
const ANOTHER_CONST = CONSTANT.'; Goodbye World';
echo ANOTHER_CONST;

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

// Works as of PHP 7
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.

Note:
Constants defined using the const keyword are always case-sensitive, while constants defined using define() may be case-insensitive.