PHP: Syntax – Manual

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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 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

  • Class Constants

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