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    defined

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

    defined — 指定した名前の定数が存在するかどうかを調べる

    説明

    defined(string $constant_name): bool

    constant_name で指定した定数が定義されているかどうかを調べます。

    この関数は クラス定数 や 列挙型の case に対しても動作します。

    注意:

    変数が存在するかどうかを知りたければ、isset() を利用してください。defined() は 定数にしか適用できません。 関数が存在するかどうかを知りたければ、 function_exists() を利用してください。

    パラメータ

    constant_name

    定数名。

    戻り値

    constant_name で指定した名前の定数が定義されている 場合に true、その他の場合にfalseを返します。

    例

    例1 定数のチェック

    <?php

    /* 引用符の使い方に注意してください。これは重要です。この例では
    * 文字列 'TEST' が、定数 TEST の名前かどうかを調べています。*/
    if (defined('TEST')) {
    echo     TEST;
    }


    interface     bar {
    const     test = 'foobar!';
    }

    class     foo {
    const     test = 'foobar!';
    }

    var_dump(defined('bar::test')); // bool(true)
    var_dump(defined('foo::test')); // bool(true)

    ?>

    例2 列挙型のcase をチェックする(PHP 8.1.0 以降)

    <?php

    enum Suit
    {
    case     Hearts;
    case     Diamonds;
    case     Clubs;
    case     Spades;
    }

    var_dump(defined('Suit::Hearts')); // bool(true)

    ?>

    参考

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    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
    15 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();
    }
    up
    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
    8 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
    16 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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