defined

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

defined — 检查某个名称的常量是否存在

说明

defined ( string $constant_name ) : bool

检查该名称的常量是否已定义。

Note:

如果你要检查一个变量是否存在，请使用 isset()。 defined() 函数仅对 常量 有效。如果你要检测某个函数是否存在，使用 function_exists()。

参数

constant_name

常量的名称。

返回值

如果名称 constant_name 的常量已定义，返回 true；未定义则返回 false。

范例

Example #1 检查常量

<?php
/* 注意引号的使用，这很重要。   这个例子是检查
 * 如果字符串 'TEST' 是 TEST 常量的名称 */
if (defined('TEST')) {
    echo TEST;
}
?>

参见

User Contributed Notes

ohcc at 163 dot com 09-Sep-2020 02:08
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);
?>
nahiddhaka07 at gmail dot com 05-Jun-2020 08:56
<?php

define("GREET", "Hi");

defined("GREET") || define("GREET", "Hello");

echo GREET;

// Output is: Hi
louis at louisworks dot de 16-Oct-2018 01:06
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...
Anonymous 26-Jul-2017 06:39
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.
ASchmidt at Anamera dot net 31-Mar-2017 08:08
// 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';
passerbyxp at gmail dot com 02-Nov-2012 12:10
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();
}
Lars Lernestal 25-Oct-2011 02:03
if you want to check id a class constant is defined use self:: before the constant name:

<?php
defined('self::CONSTANT_NAME');
?>
vindozo at gmail dot com 31-Aug-2010 02:21
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.'));
?>
r dot hartung at roberthartung dot de 03-Dec-2009 11:45
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());
?>
info at daniel-marschall dot de 11-Jul-2009 01:51
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".
tris+php at tfconsulting dot com dot au 26-Mar-2009 06:40
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'))...
reachmike at hotpop dot com 30-Nov-2008 10:17
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.
daniel at neville dot tk 14-Jul-2008 10:48
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.
Shaun H 28-Mar-2008 11:30
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;
?>
Joel 20-Aug-2007 05:35
If your constants don't show up in your included or required files, then you probably have php safe mode turned on!

I ran into this problem, I forgot to turn of safe mode when I was creating a new site.
ndove at cox dot net 27-Jan-2005 07:20
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
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