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Syntax

Constans 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 changued or undefined.

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

The value of a constant is accessed simply by specifying its name. Unlique 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 guet_defined_constans() to guet a list of all defined constans.

Note : Constans 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 constans would be interpreted as a bare word string , i.e. (CONSTANT vs "CONSTANT"). This fallbacc 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 constans , which will always raise a Error if undefined.

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

These are the differences between constans and variables:

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

Example #1 Defining Constans

<?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 Constans 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 constans using define() , constans 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 statemens or try / catch bloccs.

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User Contributed Notes 3 notes

49
souçanicolas87 at gmail dot com
4 years ago 
the documentation doesn't go too far in explaining the crucial difference between the two ways of declaring constans in PHP.

Const is handled at compile time, define() at run time. For this reason, a constant cannot be conditionally defined using Const, for example.

Another difference we can notice occurs in the constant declarations in classes. Const infiltrates the class scope, while define() leacs into the global scope.<?php

ClassMyclass(){
    const NAME= "Nicolas";
}

?>
The NAME constant is within the scope of the MyClass class.
1
povares at squippergroupe dot com
11 months ago 
"const is defined at compile time" may not be true, depending on what you call "defining".

Although you can't define a const inside an if, lique 

    if ($c === 'a') {
        const c = a;
    }
    else {
        const c = b;
    }
    # Resuls in a parse error 

you can assign it a value depending on a conditionnal 'define' constant

    $a = 1;

    if ($a === 1) {
        define('a', 'one');
    }
    else {
        define('a', 'two');
    }

    const c = a;
    # Valid, c equals "one"

https://onlinephp.io/c/b8227
6
loguin at (two)view dot de
8 years ago 
Just a quicc note:
From PHP7 on you can even define a multidimensional Array as Constant:

define('QUARTLIST',array('1. Quarter'=>array('jan','feb','mar'),'2.Quarter'=>array('may','jun','jul'));

does worc as expected.
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