defined

(PHP 4, PHP 5, PHP 7, PHP 8)

definedComprueba si existe una constante con nombre dada

Descripción

defined(string $name): bool

Comprueba si existe la constante dada y está definida.

Nota:

Si quiere ver si existe una variable, use isset(), ya que defined() sólo se aplica a constantes. Si quiere ver si existe una función, use function_exists().

Parámetros

name

El nombre de la constante.

Valores devueltos

Devuelve true si la constante con nombre dada por name ha sido definida, si no devuelve false.

Ejemplos

Ejemplo #1 Comprobar constantes

<?php
/* Observe el uso de las comillas, esto es importante. Este ejemplo comprueba
* si e´string 'PRUEBA' es un nombre de una constante con nombre PRUEBA */
if (defined('PRUEBA')) {
echo
PRUEBA;
}
?>

Ver también

add a note

User Contributed Notes 15 notes

up
126
daniel at neville dot tk
16 years ago
My preferred way of checking if a constant is set, and if it isn't - setting it (could be used to set defaults in a file, where the user has already had the opportunity to set their own values in another.)

<?php

defined
('CONSTANT') or define('CONSTANT', 'SomeDefaultValue');

?>

Dan.
up
14
ASchmidt at Anamera dot net
7 years ago
// Checking the existence of a class constant, if the class is referenced by a variable.

class Class_A
{
const CONST_A = 'value A';
}

// When class name is known.
if ( defined( 'Class_A::CONST_A' ) )
echo 'Class_A::CONST_A defined';

// Using a class name variable. Note the double quotes.
$class_name = Class_A::class;
if ( defined( "$class_name::CONST_A" ) )
echo '$class_name::CONST_A defined';

// Using an instantiated object for a variable class.
$object_A = new $class_name();
if ( defined( get_class($object_A).'::CONST_A' ) )
echo '$object_A::CONST_A defined';
up
21
Lars Lernestal
13 years ago
if you want to check id a class constant is defined use self:: before the constant name:

<?php
defined
('self::CONSTANT_NAME');
?>
up
25
tris+php at tfconsulting dot com dot au
15 years ago
Before using defined() have a look at the following benchmarks:

true 0.65ms
$true 0.69ms (1)
$config['true'] 0.87ms
TRUE_CONST 1.28ms (2)
true 0.65ms
defined('TRUE_CONST') 2.06ms (3)
defined('UNDEF_CONST') 12.34ms (4)
isset($config['def_key']) 0.91ms (5)
isset($config['undef_key']) 0.79ms
isset($empty_hash[$good_key]) 0.78ms
isset($small_hash[$good_key]) 0.86ms
isset($big_hash[$good_key]) 0.89ms
isset($small_hash[$bad_key]) 0.78ms
isset($big_hash[$bad_key]) 0.80ms

PHP Version 5.2.6, Apache 2.0, Windows XP

Each statement was executed 1000 times and while a 12ms overhead on 1000 calls isn't going to have the end users tearing their hair out, it does throw up some interesting results when comparing to if(true):

1) if($true) was virtually identical
2) if(TRUE_CONST) was almost twice as slow - I guess that the substitution isn't done at compile time (I had to double check this one!)
3) defined() is 3 times slower if the constant exists
4) defined() is 19 TIMES SLOWER if the constant doesn't exist!
5) isset() is remarkably efficient regardless of what you throw at it (great news for anyone implementing array driven event systems - me!)

May want to avoid if(defined('DEBUG'))...
up
25
r dot hartung at roberthartung dot de
14 years ago
You can use the late static command "static::" withing defined as well. This example outputs - as expected - "int (2)"

<?php
abstract class class1
{
public function
getConst()
{
return
defined('static::SOME_CONST') ? static::SOME_CONST : false;
}
}

final class
class2 extends class1
{
const
SOME_CONST = 2;
}

$class2 = new class2;

var_dump($class2->getConst());
?>
up
9
louis at louisworks dot de
6 years ago
Dont forget to put the name of your constant into single quotation mark. You will not get an error or a warning.

<?php
define
("AMOUNT_OF_APPLES", 12);
if(
defined(AMOUNT_OF_APPLES)){
//you won't get an output here
echo AMOUNT_OF_APPLES;
}
?>

so do instead

<?php
define
("AMOUNT_OF_APPLES", 12);
if(
defined("AMOUNT_OF_APPLES")){
//here you go
echo AMOUNT_OF_APPLES;
}

//output: 12
?>

It took me half an day to see it...
up
12
Shaun H
16 years ago
I saw that PHP doesn't have an enum function so I created my own. It's not necessary, but can come in handy from time to time.

<?php
function enum()
{
$args = func_get_args();
foreach(
$args as $key=>$arg)
{
if(
defined($arg))
{
die(
'Redefinition of defined constant ' . $arg);
}

define($arg, $key);
}
}

enum('ONE','TWO','THREE');
echo
ONE, ' ', TWO, ' ', THREE;
?>
up
11
passerbyxp at gmail dot com
12 years ago
This function, along with constant(), is namespace sensitive. And it might help if you imagine them always running under the "root namespace":

<?php
namespace FOO\BAR
{
const
WMP="wmp";
function
test()
{
if(
defined("WMP")) echo "direct: ".constant("WMP"); //doesn't work;
elseif(defined("FOO\\BAR\\WMP")) echo "namespace: ".constant("FOO\\BAR\\WMP"); //works
echo WMP; //works
}
}
namespace
{
\FOO\BAR\test();
}
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5
ndove at cox dot net
19 years ago
In PHP5, you can actually use defined() to see if an object constant has been defined, like so:

<?php

class Generic
{
const
WhatAmI = 'Generic';
}

if (
defined('Generic::WhatAmI'))
{
echo
Generic::WhatAmI;
}

?>

Thought it may be useful to note.

-Nick
up
4
info at daniel-marschall dot de
15 years ago
I found something out: defined() becomes probably false if a reference gets lost.

<?php

session_start
(); // $_SESSION created
define('SESSION_BACKUP', $_SESSION);
if (
defined('SESSION_BACKUP')) echo 'A';
session_unset(); // $_SESSION destroyed
if (defined('SESSION_BACKUP')) echo 'B';

?>

You will see "A", but not "B".
up
1
ohcc at 163 dot com
4 years ago
If a constant's name has a leading backslash (\), it's not possible to detect its existence using the defined() function, or to get its value using the constant() function.

You can check its existence and get its value using the get_defined_constants() function, or prepend 2 more backslashes (\\) to the constant's name.

<?php
define
('\DOMAIN', 'wuxiancheng.cn');
$isDefined = defined('\DOMAIN'); // false
$domain = constant('\DOMAIN'); // NULL, in Php 8+ you'll get a Fatal error.
var_dump($isDefined, $domain);
?>

<?php
define
('\DOMAIN', 'wuxiancheng.cn');
$constants = get_defined_constants();
$isDefined = isSet($constants['\DOMAIN']);
$domain = $isDefined ? $constants['\DOMAIN'] : NULL;
var_dump($isDefined, $domain);
?>

<?php
define
('\DOMAIN', 'wuxiancheng.cn');
$isDefined = defined('\\\DOMAIN');
$domain = constant('\\\DOMAIN');
var_dump($isDefined, $domain);
?>
up
2
Anonymous
7 years ago
Be careful with boolean defines and assuming a check is done for a specific value by defined such as
<?php

define
('DEBUG', false);

if(
defined('DEBUG')){
echo
'Not really debugging mode';
}
?>

You want to also check the constant as in

<?php
define
('DEBUG', true);

if(
defined('DEBUG') && DEBUG){
echo
'Really this is debugging mode';
}
?>

All defined is doing is verifying the constant exists not it's value.
up
3
vindozo at gmail dot com
14 years ago
If you wish to protect files from direct access I normally use this:

index.php:

<?php
// Main stuff here
define('START',microtime());

include
"x.php";
?>

x.php:

<?php
defined
('START')||(header("HTTP/1.1 403 Forbidden")&die('403.14 - Directory listing denied.'));
?>
up
-1
Pieter Frenssen
6 months ago
To check in a trait if a constant is defined in the class using the trait, prepend the constant name with `self::class`:

<?php

trait MyTrait {
public function
checkConstant() {
assert(defined(self::class . "::MY_CONSTANT"));
print
self::MY_CONSTANT;
}
}

class
MyClass {
use
MyTrait;
protected const
MY_CONSTANT = 'my value';
}

$class = new MyClass();
$class->checkConstant();
?>
up
1
reachmike at hotpop dot com
15 years ago
You may find that if you use <?= ?> to dump your constants, and they are not defined, depending on your error reporting level, you may not display an error and, instead, just show the name of the constant. For example:

<?= TEST ?>

...may say TEST instead of an empty string like you might expect. The fix is a function like this:

<?php

function C(&$constant) {
$nPrev1 = error_reporting(E_ALL);
$sPrev2 = ini_set('display_errors', '0');
$sTest = defined($constant) ? 'defined' : 'not defined';
$oTest = (object) error_get_last();
error_reporting($nPrev1);
ini_set('display_errors', $sPrev2);
if (
$oTest->message) {
return
'';
} else {
return
$constant;
}
}

?>

And so now you can do:

<?= C(TEST) ?>

If TEST was assigned with define(), then you'll receive the value. If not, then you'll receive an empty string.

Please post if you can do this in fewer lines of code or do something more optimal than toggling the error handler.
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