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Une constante est un identifiant (un nom) qui représente une valeur simple. Comme son nom le suggère, cette valeur ne peut jamais être modifiée durant l'exécution du script (sauf les constantes magiques). Par défaut, le nom d'une constante est sensible à la casse. Par convention, les constantes sont toujours en majuscules.

Les noms de constantes suivent les mêmes règles que n'importe quel nom en PHP. Un nom de constante valide commence par une lettre ou un souligné, suivi d'un nombre quelconque de lettres, chiffres ou soulignés. Sous forme d'expression régulière, cela peut s'exprimer comme ceci : [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Astuce

Vous pourriez également avoir besoin de jeter un oeil sur Guide de nommage de l'espace utilisateur.

Exemple #1 Noms valides et invalides pour les constantes

// Noms valides
define("FOO",     "something");
define("FOO2",    "something else");
define("FOO_BAR""something more");

// Noms invalides
define("2FOO",    "something");

// Ce nom est valide, mais évitez-le:
// PHP peut un jour fournir une constante magique nommée
// ainsi, ce qui va corrompre vos scripts.
define("__FOO__""something");

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Note: Dans cette documentation, une lettre peut être un des caractères suivants : de a à z, de A à Z et tous les caractères ASCII de 127 à 255 (0x7f-0xff).

Tout comme les superglobals, les constantes sont accessibles de manière globale. Vous pouvez les définir n'importe où, et y accéder depuis n'importe quelle fonction. Pour plus d'informations sur le contexte, lisez la section du manuel sur la portée des variablesBone Womens Oryx Grafton Old Gringo gqPwHZAZ.

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User Contributed Notes 11 notes

wbcarts at juno dot com
6 years ago
11/14/2016 - note updated by sobak
-----

CONSTANTS and PHP Class Definitions

Using "define('MY_VAR', 'default value')" INSIDE a class definition does not work as expected. You have to use the PHP keyword 'const' and initialize it with a scalar value -- boolean, int, float, string (or array in PHP 5.6+) -- right away.


define
('MIN_VALUE', '0.0');   // RIGHT - Works OUTSIDE of a class definition.
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define('MAX_VALUE', '1.0');   // RIGHT - Works OUTSIDE of a class definition.

//const MIN_VALUE = 0.0;         RIGHT - Works both INSIDE and OUTSIDE of a class definition.
//const MAX_VALUE = 1.0;         RIGHT - Works both INSIDE and OUTSIDE of a class definition.

class Constants
{
 
//define('MIN_VALUE', '0.0');  WRONG - Works OUTSIDE of a class definition.
  //define('MAX_VALUE', '1.0');  WRONG - Works OUTSIDE of a class definition.

 
const MIN_VALUE = 0.0;      // RIGHT - Works INSIDE of a class definition.
 
const MAX_VALUE = 1.0;      // RIGHT - Works INSIDE of a class definition.

 
public static function Modern Wedge Skechers Black Maze Rumblers Women's Cali OwwXxS4tqgetMinValue()
  {
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    return
self::MIN_VALUE;
  }

  public static function
getMaxValue()
  {
    return
self::MAX_VALUE;
  }
}

?>

#Example 1:
You can access these constants DIRECTLY like so:
* type the class name exactly.
* type two (2) colons.
* type the const name exactly.

#Example 2:
Because our class definition provides two (2) static functions, you can also access them like so:
* type the class name exactly.
* type two (2) colons.
* type the function name exactly (with the parentheses).


#Example 1:
$min = Constants::Black Black Mens Zipper Rieker M Boots MIN_VALUE;
$max = Constants::MAX_VALUE;

#Example 2:
$min = Constants::getMinValue();
$max = Constants::getMaxValue();

?>

Once class constants are declared AND initialized, they cannot be set to different values -- that is why there are no setMinValue() and setMaxValue() functions in the class definition -- which means they are READ-ONLY and STATIC (shared by all instances of the class).
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11
Raheel Khan
3 years ago
class constant are by default public in nature but they cannot be assigned visibility factor and in turn gives syntax error


class constants {

    const
MAX_VALUE = Black Zipper Boots Black Mens M Rieker 10;
        public const
MIN_VALUE =1;

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}

// This will work
echo constants::MAX_VALUE;

// This will return syntax error
echo constants::MIN_VALUE;
?>
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15
katana at katana-inc dot com Stan Core Women's Originals Core adidas Pink Smith White Pink YxwEqg5Pg
16 years ago
Warning, constants used within the heredoc syntax (http://www.php.net/manual/en/language.types.string.php) are not interpreted!

Editor's Note: This is true. PHP has no way of recognizing the constant from any other string of characters within the heredoc block.
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13
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15 years ago
I find using the concatenation operator helps disambiguate value assignments with constants. For example, setting constants in a global configuration file:

define('LOCATOR',   "/locator");
define('CLASSES',   LOCATOR."/code/classes");
defineBlack M Rieker Boots Mens Zipper Black ('FUNCTIONS', LOCATOR."/code/functions");
define('USERDIR',   LOCATOR."/user");
?>

Later, I can use the same convention when invoking a constant's value for static constructs such as require() calls:

require_once(FUNCTIONS."/database.fnc");
require_once(
FUNCTIONS."/randchar.fnc");
?>

as well as dynamic constructs, typical of value assignment to variables:

$userid  = randchar(8,'anc','u');
$usermap = USERDIR."/".$userid.".png";
?>

The above convention works for me, and helps produce self-documenting code.

-- Erich
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10
hafenator2000 at yahoo dot com Natural Fabric Wedge Strap Quarter Lucca Women's Sandal Cradles Walking Xv0q7F8q
13 years ago
PHP Modules also define constants.  Make sure to avoid constant name collisions.  There are two ways to do this that I can think of.
First: in your code make sure that the constant name is not already used.  ex. if (! defined("CONSTANT_NAME"Zipper Boots Black Rieker M Mens Black )) {Define("CONSTANT_NAME","Some Value"); } ?>  This can get messy when you start thinking about collision handling, and the implications of this.
Second: Use some off prepend to all your constant names without exception  ex. ("SITE_CONSTANT_NAME","Some Value"); ?>

Perhaps the developers or documentation maintainers could recommend a good prepend and ask module writers to avoid that prepend in modules.
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14
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13 years ago
An undefined constant evaluates as true when not used correctly. Say for example you had something like this:

settings.php
// Debug mode
define('DEBUG',false);
?>

test.php
include('settings.php');

if (
DEBUG) {
  
// echo some sensitive data.
}
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?>

If for some reason settings.php doesn't get included and the DEBUG constant is not set, PHP will STILL print the sensitive data. The solution is to evaluate it. Like so:

settings.php
// Debug mode
define('DEBUG',0);
?>

test.php
include('settings.php');

if (
DEBUG == 1) {
  
// echo some sensitive data.
}
?>

Now it works correctly.
gried at NOSPAM dot nsys dot by
2 years ago
Lets expand comment of 'storm' about usage of undefined constants. His claim that 'An undefined constant evaluates as true...' is wrong and right at same time. As said further in documentation ' 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...'. So yeah, undefined global constant when accessed directly will be resolved as string equal to name of sought constant (as thought PHP supposes that programmer had forgot apostrophes and autofixes it) and non-zero non-empty string converts to True.

There are two ways to prevent this:
1. always use function constant('CONST_NAME') to get constant value (BTW it also works for class constants - constant('CLASS_NAME::CONST_NAME') );
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2. use only class constants (that are defined inside of class using keyword const) because they are not converted to string when not found but throw exception instead (Fatal error: Undefined class constant).
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5
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11 years ago
If you are looking for predefined constants like
* PHP_OS (to show the operating system, PHP was compiled for; php_uname('s') might be more suitable),
* DIRECTORY_SEPARATOR ("\\" on Win, '/' Linux,...)
* PATH_SEPARATOR (';' on Win, ':' on Linux,...)
they are buried in 'Predefined Constants' under 'List of Reserved Words' in the appendix:
http://www.php.net/manual/en/reserved.constants.php
while the latter two are also mentioned in 'Directory Functions'
http://www.php.net/manual/en/ref.dir.php
jcastromail at yahoo dot es
10 months ago
Performance of constants.  PHP 7.1.10 32 bits (Opcache active, windows 10 i7-64bits) but apparently the trends is the same with the 5.x

using a constant declared by DEFINE('CNS',value) : 0.63575601577759s
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using a constant declared by const CNS=value : 0.61372208595276s
using a variable declared by $v=value : 0.51184010505676s

In average, the use of DEFINE and CONST is around the same with some sightly  better performance of CONST instead of DEFINE. However, using a variable is around 10-50% better than to use a constant.  So, for a performance intensive task, constant is not the best option.

$p1=microtime(true);
$x=0;
for($i=0;$i<50000000;$i++) {
    $x+=CNS;
}
$p2=microtime(true);
php at webflips dot net
4 years ago
It is perfectly valid to use a built-in PHP keyword as a constant name - as long as you the constant() function to retrieve it later:
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define('echo', 'My constant value');

echo
constant('echo'); // outputs 'My constant value'
?>
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-11
xs2ilyas at gmail dot com
1 year ago
When we start a constant name with space, it doesn't produce any error.

  define (" YEAR"2000)
  echo
"success";
?>

output: success

But when we call this constant, it produce error.

  define (" YEAR"2000)
  echo
YEAR;
?>

output: E_NOTICE :  type 8 -- Use of undefined constant YEAR - assumed 'YEAR' -- at line 4 YEAR