Last updated: Fri, 20 Jun 2008

Les constantes

Sommaire

Constantes magiques

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 (les constantes magiques __FILE__ et __LINE__ sont les seules exceptions). Le nom d'une constante est sensible à la casse, par défaut. 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 lettre, 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


<?php
// 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");

?>

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 la 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 les variables de contexte.

Syntaxe

Vous pouvez définir une constante en utilisant la fonction define(). Une fois qu'une constante est définie, elle ne peut jamais être modifiée, ou détruite.

Seuls les types de données scalaires peuvent être placés dans une constante : c'est à dire les types booléen, entier, double et chaîne de caractères (soit bool, entier, double et string). Les constantes de type resource ne peuvent pas être définies.

Vous pouvez accéder à la valeur d'une constante en spécifiant simplement son nom. Contrairement aux variables, vous ne devez PAS préfixer le nom de la constante avec $. Vous pouvez aussi utiliser la fonction constant(), pour lire dynamiquement la valeur d'une constante, dont vous obtenez le nom dynamiquement (retour de fonction, par exemple). Utilisez la fonction get_defined_constants() pour connaître la liste de toutes les constantes définies.

Note: Les constantes et les variables globales utilisent deux espaces de noms différents. Ce qui implique que TRUE et $TRUE sont généralement différents (en tous cas, ils peuvent avoir des valeurs différentes).

Si vous utilisez une constante non définie, PHP considère que vous uniquement le nom de la constante elle-même, comme si vous l'appeliez comme étant une chaîne de caractères (CONSTANT vs "CONSTANT"). Une alerte de type E_NOTICE sera émise lorsque ce cas se produit. Lisez également l'entrée du manuel qui explique pourquoi $foo[bar] est faux (tant que vous ne définissez pas bar comme étant une constante). Si vous voulez simplement vérifier qu'une constante est définie, utilisez la fonction defined().

Il y a des différences entre les constantes et les variables :

Les constantes ne commencent pas par le signe ($);
Les constantes sont définies et accessibles à tout endroit du code, globalement.
Les constantes ne peuvent pas être redéfinies ou indéfinies une fois qu'elles ont été définies.
Les constantes ne peuvent contenir que des scalaires.

Exemple #2 Définir une constante


<?php
  define("CONSTANTE", "Bonjour le monde.");
  echo CONSTANTE; // affiche "Bonjour le monde."
  echo Constante; // affiche "Constante" et une note.
?>

Voir aussi les constantes de classe.

Constantes magiques

Variables externes à PHP

Last updated: Fri, 20 Jun 2008

add a note User Contributed Notes
Les constantes

ian at willis dot org dot uk
16-May-2008 12:25


Another difference - you cannot use a constant to reference a function.



<?php



function ccc()

{

   echo "CCC\n";

}



function vvv()

{

   echo "VVV\n";

}



define('cfunc', 'ccc');

$vfunc = 'vvv';



$vfunc();   // Works

cfunc();    // Fails



?>



Displays:



VVV



Fatal error: Call to undefined function cfunc() in /tmp/test.php on line 17

sgbohu at ~NOSPAM~gmail dot com
22-Apr-2008 01:25


In response to tudor at tudorholton dot com

10-Jul-2007 02:15



The error message does not give incorrect information.



The error indeed is that you did not quote the constant name and PHP tries therefore to use it as a constant. 

Since it is not defined, PHP assumes that you meant to quote it and evaluates it as a string.



While this is invalid :

<?php



define( MY_CONST, 'blah');



?>



This will work :

<?php



define( 'CONST_NAME', 'MY_CONST' );

define( CONST_NAME, 'blah'); 



var_dump( CONST_NAME ); // output : string(8) "MY_CONST"

var_dump( MY_CONST ); // output : string(4) "blah"



?>

ben at bendodson dot com
27-Nov-2007 12:14


I recently found I needed a way of retrieving the value of a constant dynamically - e.g. trying to find the value of FOO_BAR by passing 'FOO_' . $someVariableWithValueBAR.  I came up with the following solution:



<?php



define('FOO_BAR','It works!');

define('FOO_FOO_BAR','It works again!');



// prints 'It works!'

$changing_variable = 'bar';

echo constant('FOO_' . strtoupper($changing_variable));



// prints 'It works again!'

$changing_variable = 'foo_bar';

echo constant('FOO_' . strtoupper($changing_variable));



?>



Note the use of strtoupper() as constants should be defined in uppercase for good practice - feel free to remove if you have constants defined in lowercase or you can set $changing_variable as uppercase.



Might be of some use to someone!

Sinured
30-Jul-2007 08:39


Ah, I forgot to point that out in my previous note:



> "Constants may only evaluate to scalar values."



Currently, resources are "abstract datatypes based on integers".

Though this may change in the future, and is_scalar() rejects resources as non-scalar, you can define resource constants, like

<?php define('DB', mysql_connect());?>

(think of STDIN, STDOUT and STDERR).

Resource constants will still remain resources; try var_dump().

Sinured
19-Jul-2007 09:27


> "A valid constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores."



This rules only applies when using constants like <?php echo CONSTANT;?> to not confuse the parser - PHP will be happy with any string for a constant name (define).



<?php

define('Red herring!', '<°)))><');

#echo Red herring!;

// The above example will not work, the parser will get

// angry seeing a whitespace where it shouldn’t be.



// Let’s have that with constant():

echo constant('Red herring!'); // <°)))><

?>



You could go on with newlines, tabs and stuff... that is, however, crude coding style.

tudor at tudorholton dot com
10-Jul-2007 02:15


Note that constant name must always be quoted when defined.



e.g.

define('MY_CONST','blah') - correct

define(MY_CONST,'blah') - incorrect



The following error message also indicates this fact:

Notice:  Use of undefined constant MY_CONST - assumed 'MY_CONST' in included_script.php on line 5



Note the error message gives you some incorrect information.   'MY_CONST' (with quotes) doesn't actually exist anywhere in your code.  The error _is_ that you didn't quote the constant when you defined it in the 'assumed' file.

Andreas R.
30-Apr-2007 04:19


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

pdenny at magmic dot com
18-Feb-2007 06:46


Note that constants can also be used as default argument values

so the following code:



  define('TEST_CONSTANT','Works!');

  function testThis($var=TEST_CONSTANT) {

      echo "Passing constants as default values $var";

  }

  testThis();



will produce :



Passing constants as default values Works!



(I tried this in both PHP 4 and 5)

dexen at google dot me dot up
05-Sep-2006 01:02


1) Constants are invaluable when you want to be sure that *nobody*  changes your important piece of data through lifetime of script -- especially when you're developing in team -- as this can cause strange, hard to track bugs. 



2) Using constants is prefered over ``magic values'', as it leads to self-documenting code. Also saves you from scanning and tweaking tens of files should the value ever change.

Consider example: <?php 

if ( $headers['code'] = 505 ) { //wth is 505? What do following code do? ?>

versus: <?php 

if ( $headers['code'] = HTTP_VERSION_NOT_SUPPORTED ) {

   $this->useHttp = '1.0'; ?>



In response to ``kencomer'':

3) Why not to use <?php

define( 'DEBUG', FALSE );

define( 'DEBUG', TRUE ); ?>

and comment one of them out as needed when developing/deploying?

That'd save a lot of ugly ``if ( defined( 'DEBUG' ) && DEBUG ) {}''.



4) For debugging toggled on/off you pretty often want to use assert() anyway. You're free to turn it on/off at any moment (thou you better do it only once ;) ). assert() gives some nice details upon failed assertion, like file/line/function and context (that's invaluable!)

martin at larsen dot dk
23-Feb-2006 11:24


I find variables much more flexible than constants because variables can be used inside quotes and heredocs etc. Especially for language systems, this is nice.



As stated in one of the previous notes, there is no speed penalty by using variables. However, one issue is that you risc name collision with existing variables. When implementing a language system I simply found that adding a prefix to all the variables was the way to go, for example:



$LNG_myvar1 = "my value";



That is easier and performs faster than using arrays like



$LNG['myvar'] = "my value";



As a final note, implementing a new superglobal in PHP would make using constants much more beneficial. Then it could be used in qoutes like this:



"The constant myconst has the value $CONSTANTS[myconst] !"

anj at aps dot anl dot gov
20-Dec-2005 05:42


It is possible to define constants that have the same name as a built-in PHP keyword, although subsequent attempts to actually use these constants will cause a parse error. For example in PHP 5.1.1, this code



    <?php

    define("PUBLIC", "Hello, world!");

    echo PUBLIC;

    ?>



gives the error



    Parse error: syntax error, unexpected T_PUBLIC in test.php on line 3



This is a problem to be aware of when converting PHP4 applications to PHP5, since that release introduced several new keywords that used to be legal names for constants.

kencomer at NOSPAM dot kencomer dot com
14-Sep-2005 02:38


Being a belt and suspenders person, when I use a constant to do flow control (i.e., using constants to determine which version of a section of the program should be used), I always use something like:



if ( defined('DEBUG') && TRUE===DEBUG )



If you accidentally use DEBUG somewhere before it is defined, PHP will create a new constant called DEBUG with the value 'DEBUG'. Adding the second comparison will prevent the expression from being TRUE when you did not intentionally create the constant. For the constant DEBUG, this would rarely be a problem, but if you had (e.g.) a constant used to determine whether a function was created using case-sensitive comparisons, an accidental creation of the constant IGNORE_CASE having the value 'IGNORE_CASE' could drive you up the wall trying to find out what went wrong, particularly if you had warnings turned off.



In almost all code I write, I put this function definition in my configuration section:



if (!function_exists("debug_print")) {

  if ( defined('DEBUG') && TRUE===DEBUG ) {

    function debug_print($string,$flag=NULL) {

      /* if second argument is absent or TRUE, print */

      if ( !(FALSE===$flag) )

        print 'DEBUG: '.$string . "\n";

    }

  } else {

    function debug_print($string,$flag=NULL) {

    }

  }

}



Then, in my code, I'll sprinkle liberal doses of debug code like :



define("DEBUG_TRACK_EXAMPLE_CREATION",FALSE);

class Example extends Something {

  __construct($whatever) {

    debug_print( "new instance of Example created with '$whatever'\n",DEBUG_TRACK_EXAMPLE_CREATION);

  }

}



and :



debug_print("finished init.\n")



In the first case, I would not want to see that message every time I went into DEBUG mode, so I made it a special case. The second case is always printed in DEBUG mode. If I decide to turn everything on, special cases and all, all I have to do is comment out the "if" line in debug_print() and presto magicko! It costs a little and gains a lot.



As another belt-and-suspenders aside, notice that, unlike most people, I put the language constant (e.g.,TRUE, "string", etc.) on the left side of the comparison. By doing that, you can never accidentally do something like 

  if ( $hard_to_find_error="here" )



because you always write it as 

  if ( "here"==$no_error )



or, if you got it wrong,

  if ( "here"=$easy_to_find_parse_error )

a dot eibach at gmx dot net
01-Sep-2005 11:11


It took me almost 30 minutes to find out what was wrong in my code. I thought I had defined all constants correctly: correct quotes, and whatnot.

The problem: I am a C programmer and I used #define with the preprocessor hash sign! No effect, naturally.

So if you happen to come from C world and you program PHP, *DO NOT* use the preprocessor hash as you're used to in C.

Angelina Bell
25-Jul-2005 09:39


It is so easy to create a constant that the php novice might do so accidently while attempting to call a function with no arguments.  For example:

<?php

function LogoutUser(){

// destroy the session, the cookie, and the session ID

  blah blah blah;

  return true;

}

function SessionCheck(){

  blah blah blah;

// check for session timeout

...

    if ($timeout) LogoutUser;  // should be LogoutUser();

}

?>



OOPS!  I don't notice my typo, the SessionCheck function

doesn't work, and it takes me all afternoon to figure out why not!



<?php

LogoutUser;

print "new constant LogoutUser is " . LogoutUser;

?>

ck
27-May-2005 04:23


Re: Storm.

I ran that code (in PHP4)

<?php

if (DEBUG) {

   // echo some sensitive data.

}

?>

 and saw this warning:

"Use of undefined constant DEBUG - assumed 'DEBUG'"



A clearer workaround is to use

<?php

if (defined('DEBUG')) {

   // echo some sensitive data.

}

?>

Thanks for pointing out this big gotcha.



Another reason to turn on warnings during testing.  Good web servers are set up to suppress warning and error output to the browser, so this is handy:

<?php

if (defined('DEBUG')) {

  error_reporting(E_ALL);

  set_error_handler('debug_ErrorHandler');

}

function debug_ErrorHandler($errno, $errstr, $errfile, $errline) {

  print("PHP Error [$errno] [$errstr] at $errline in $errfile.<br>");

}

?>

hafenator2000 at yahoo dot com
21-Apr-2005 11:09


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. <?php if (! defined("CONSTANT_NAME")) { 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. <?php Define("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.

storm
18-Apr-2005 06:54


An undefined constant evaluates as true when not used correctly. Say for example you had something like this:



settings.php

<?php

// Debug mode

define('DEBUG',false);

?>



test.php

<?php

include('settings.php');



if (DEBUG) {

   // echo some sensitive data.

}

?>



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

<?php

// Debug mode

define('DEBUG',0);

?>



test.php

<?php

include('settings.php');



if (DEBUG == 1) {

   // echo some sensitive data.

}

?>



Now it works correctly.

Charles
12-Jan-2005 10:50


To clarify from the previous post:



When you define a constant, it becomes fixed at that point and is immutable. You can add variables - but the constant becomes the contents of that variable when the define is evaluated. If you try:



define( "_A_TEXT" , "The value is " . $arr[$i] );



It would be evaluated ONCE with the current value of the $i index of array $arr. As the post pointed out, this is probably not what you want. You can easily create:



define( "_A_TEXT" , "The value is ");

....

echo _A_TEXT . $arr[$i];



Which would give you what you wanted: the constant string with the contents of the array appended.

the_coder at colina2004 dot com
24-Jun-2004 10:42


I'm currently working on a site that has got to have two languages, and I wanted to use define's in functions to make everything simpler.



However, I ran into a problem. PHP doesn't recognize the variable in:

define("constantName", "This is an array variable - {$array[$i][2]}");



I can't use that in a for cycle, like I wanted to:



for ($i = 0; $i < count($array); $i++) {

echo constantName . "<br />"

}



The method I found (I think it's been mentioned before) is to:



define("constantName", "This is an array variable - %s");



And then:



for ($i = 0; $i < count($array); $i++) {

printf(constantName, $array[$i][2]);

}

kumar at farmdev
26-Oct-2003 02:59


before embarking on creating a language system I wanted to see if there was any speed advantage to defining language strings as constants vs. variables or array items.  It is more logical to define language strings as constants but you have more flexibility using variables or arrays in your code (i.e. they can be accessed directly, concatenated, used in quotes, used in heredocs whereas constants can only be accessed directly or concatenated).



Results of the test:

declaring as $Variable is fastest

declaring with define() is second fastest

declaring as $Array['Item'] is slowest



=======================================

the test was done using PHP 4.3.2, Apache 1.3.27, and the ab (apache bench) tool.

100 requests (1 concurrent) were sent to one php file that includes 15 php files each containing 100 unique declarations of a language string.



Example of each declaration ("Variable" numbered 1 - 1500):

<?php

$GLOBALS['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";

?>

<?php

define('Variable1' , "A whole lot of text for this variable as if it were a language string containing a whole lot of text");

?>

<?php

$GLOBALS['CP_Lang']['Variable1'] = "A whole lot of text for this variable as if it were a language string containing a whole lot of text";

?>



Here are the exact averages of each ab run of 100 requests (averages based on 6 runs):

variable (24.956 secs)

constant (25.426 secs)

array (28.141)



(not huge differences but good to know that using variables won't take a huge performance hit)

ewspencer at industrex dot com
18-Aug-2003 03:30


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");

define('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

php-comment-2003-july-24 at ryandesign dot de
24-Jul-2003 04:04


Late reply to fmmarzoa at gmx dot net: You're better off using sprintf format and defining your strings like this:



define('strArticleDescr', 'Published by %1$s on %2$s in %2$s');



It's more standard than what you're doing. Then instead of outputting it using an eval, do this:



echo sprintf(strArticleDescr, $article_author, $article_date, $article_lang_name');



And even better for i18n and l10n, don't use defines; use gettext. See the PHP manual section on gettext and the GNU gettext website. Gettext requires some modification of the way you think about strings but I find it worthwhile to make that adjustment.

Mike Powell
25-Mar-2003 05:46


In response to the notes above about variable references in constants, double quotes isn't a proper solution because it parses the variable at the time the constant is defined. The desired behavior is to have the variables parsed at the time the constant is referenced, and this behavior can really only be achieved by using eval(), as described above.

gv (at) damnsw (dot) net
06-Nov-2002 05:08


fmmarzoa: In PHP 4.2.2/CLI, I had no problem setting define()'s to the contents of variables:



<?

    $foo = "PHP";

    define( "bar", "$foo is a good thing." );

    print bar;

?>



Will print "PHP is a good thing.".



A notable difference, however, between my example and yours is your use of single-quotes.  Strings in single quotes (') will not be expanded: 



print '$foo'; 



Will print '$foo', not the contents of $foo.



http://www.php.net/manual/en/language.types.string.php



--gv

alan at akbkhome dot com
23-Mar-2002 06:08


The __FILE__ constant in 4.2rc1 (CLI) will return the location of script specified to be run, rather than the absolute file.



eg. /usr/bin/phpmole (a softlink to /usr/lib/php/phpmole/phpmole.php)



started like this

bash#/usr/bin/phpmole 

 

the line echo __FILE__ in phpmole.php will output /usr/bin/phpmole - in the CGI it would have returned /usr/lib/php/phpmole/phpmole.php



the workaround is to check for links!!

$f = __FILE__;

if (is_link($f)) $f = readlink($f);

katana at katana-inc dot com
25-Feb-2002 08:53


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.

afuse at yahoo dot com
11-Jun-2001 07:42


The pre-defined constant '__FILE__' does not work in same way at every version of PHP.





Some version of PHP has the relative path, and some other has the absolute path on __FILE__ constant..





Please be carefull in use..





[PS]


I have not tested at all versions of PHP but the version of 4.04pl.. and 4.05 are certainly not working in same way..  If you want to see that bug(?), I can show you an example.

silvein at sonique dot com
24-Jan-2001 02:54


It may be useful to note that, in php4 (what version this started I don't know, but it didn't do it before we upgraded to php4) __FILE__ will follow symlinks to the origional file.

tom dot harris at home dot com
05-Aug-2000 02:44


To get a full path (the equivalent of something like "__PATH__") use 


dirname($SCRIPT_FILENAME)


to get the directory name of the called script and


dirname(__FILE__)


to get the directory name of the include file.

add a note

Constantes magiques

Variables externes à PHP

Last updated: Fri, 20 Jun 2008