PHP 5.6.14 is available


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. Once a constant is defined, it can never be changed or undefined.

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"). An error of level E_NOTICE will be issued when this happens. 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

("CONSTANT""Hello world.");
CONSTANT// outputs "Hello world."
echo Constant// outputs "Constant" and issues a notice.

Example #2 Defining Constants using the const keyword

// Works as of PHP 5.3.0
const CONSTANT 'Hello World';


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

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

// Works as of PHP 7
define('ANIMALS', array(
ANIMALS[1]; // outputs "cat"


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 Class Constants.

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User Contributed Notes 3 notes

9 months ago
The const keyword will not allow anything that may have to be "processed":

$color = "red";
const RED = "This is the color $color"; //Doesn't work
define(strtoupper($color), "This is the color $color") // Works fine
0gb dot us at 0gb dot us
3 years ago
While most constants are only defined in one namespace, the case-insensitive true, false, and null constants are defined in ALL namespaces. So, this is not valid:

<?php namespace false;
ENT_QUOTES = 'My value';
ENT_QUOTES;//Outputs as expected: 'My value'

const FALSE = 'Odd, eh?';//FATAL ERROR! ?>

Fatal error: Cannot redeclare constant 'FALSE' in /Volumes/WebServer/ on line 5
kuzawinski dot marcin at NOSPAM dot gmail dot com
1 year ago
Actually, there is a way, to (re)declare FALSE (also: False, false, TRUE, NULL, etc.) constant, even if it is already declared:


(constant('I_DONT_EXIST')); // warning "Couldn't find constant I_DONT_EXIST"
var_dump(constant('FALSE'));  //no warning, output: bool(false)

define('FALSE', TRUE);

var_dump(constant('FALSE')); //output: bool(true)

// but...

var_dump(constant('false')); // output: bool(false)
var_dump(FALSE); // output: bool(false);

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