define

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

define — 名前を指定して定数を定義する

説明

define(string $constant_name, mixed $value, bool $case_insensitive = false): bool

実行時に、名前を指定して定数を定義します。

パラメータ

constant_name: 定数の名前。

注意:
予約語や無効な名前を使っていても、定数を define() できてしまいます。これらの値を取得するには、 constant() 関数を使うこと(だけ)しかできません。しかしながら、こういったことをするのは推奨されません。
value: 定数の値。PHP 5 では、value はスカラー値 (int、 float、string、bool あるいは null) でなければいけません。PHP 7 では配列を使うこともできます。

警告
リソース型の定数を定義することもできますが、推奨できません。予期せぬ振る舞いをする可能性があります。
case_insensitive: true を指定すると、定数は大文字小文字を区別しないようになります。デフォルトでは大文字小文字を区別します。つまり CONSTANT と Constant は別の値を表すわけです。

警告
大文字小文字を区別しない定数を定義するのは、 PHP 7.3.0 以降では推奨されなくなりました。 PHP 8.0.0 以降では、false のみを受け入れます。 true を渡すと、警告が発生します。

注意:
大文字小文字を区別しないときは、定数は小文字で格納されます。

戻り値

成功した場合に true を、失敗した場合に false を返します。

変更履歴

バージョン	説明
8.0.0	`case_insensitive` に `true` を渡すと、 `E_WARNING` が発生するようになりました。 `false` を渡すことはまだ許可されています。
7.3.0	`case_insensitive` は非推奨になりました。 8.0.0 で削除される予定です。
7.0.0	array が使えるようになりました。

例

例1 定数の定義

<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // "Hello world." を出力します
echo Constant; // "Constant" を出力し、警告が発生します

define("GREETING", "Hello you.", true);
echo GREETING; // "Hello you." を出力します
echo Greeting; // "Hello you." を出力します

// PHP 7 以降で動作します
define('ANIMALS', array(
    'dog',
    'cat',
    'bird'
));
echo ANIMALS[1]; // "cat" を出力します

?>

例2 予約された名前を定数にする

マジック定数と同じ名前の定数を定義できる 可能性 を示します。しかしながら、結果として起きる振る舞いは明らかに混乱を招くため、こんなことを実際にすることは推奨されません。

<?php
var_dump(defined('__LINE__'));
var_dump(define('__LINE__', 'test'));
var_dump(constant('__LINE__'));
var_dump(__LINE__);
?>

上の例の出力は以下となります。

bool(false)
bool(true)
string(4) "test"
int(5)

参考

defined() - 指定した名前の定数が存在するかどうかを調べる
constant() - 定数の値を返す
定数の節

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User Contributed Notes 11 notes

down

101

ravenswd at gmail dot com ¶

9 years ago

Be aware that if "Notice"-level error reporting is turned off, then trying to use a constant as a variable will result in it being interpreted as a string, if it has not been defined.

I was working on a program which included a config file which contained:

<?php
define('ENABLE_UPLOADS', true);
?>

Since I wanted to remove the ability for uploads, I changed the file to read:

<?php
//define('ENABLE_UPLOADS', true);
?>

However, to my surprise, the program was still allowing uploads. Digging deeper into the code, I discovered this:

<?php
if ( ENABLE_UPLOADS ):
?>

Since 'ENABLE_UPLOADS' was not defined as a constant, PHP was interpreting its use as a string constant, which of course evaluates as True.

down

@SimoEast on Twitter ¶

7 years ago

Not sure why the docs omit this, but when attempting to define() a constant that has already been defined, it will fail, trigger an E_NOTICE and the constant's value will remain as it was originally defined (with the new value ignored).

(Guess that's why they're called "constants".)

down

danbettles at yahoo dot co dot uk ¶

15 years ago

define() will define constants exactly as specified.  So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace.  The following examples will make it clear.

The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").

<?php
namespace test;
define('MESSAGE', 'Hello world!');
?>

The following code will define two constants in the "test" namespace.

<?php
namespace test;
define('test\HELLO', 'Hello world!');
define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!');
?>

down

Dale Landry ¶

7 years ago

With php 7 you can now define arrays. 

consider the following code:
<?php

define( "EXPLENATIVES", [1 => "Foo Bar", 2 => "Fehw Bahr", 3 => "Foo Bahr", 4 => "Fooh Bar", 5 => "Fooh Bhar", 6 => "Foo Barr", 7 => "Foogh Bar", 8 => "Fehw Barr", 9 => "Fu bar", 10 => "Foo Bahr", 11 => "Phoo Bar", 12 => "Foo Bawr", 13 => "Phooh Baughr", 14 => "Foogan Bargan", 15 => "Foo Bahre", 16 => "Fu Bahar", 17 => "Fugh Bar", 18 => "Phou Baughr"]);

//set up define methods using mixed values; both array and non-array values
define("NAVBTNS", [EXPLENATIVES, "Nouns", "Verbs", "Adjectives"]);

//function to create a dropdown menu using the EXPLENATIVES array $btn=EXPLENATIVES=assoc_array

function construct_navbar_buttons(){
    
    $btns = '<ul class="nav navbar-nav">';
                        
    foreach(NAVBTNS as $button => $btn){
        if(is_array($btn)){
            $btns .= '<li class="dropdown">
                        <a class="dropdown-toggle" data-toggle="dropdown" href="?id='.$btn.'">
                            <i class="glyphicon glyphicon-user"></i> You Dare Say? <i class="glyphicon glyphicon-collapse-down"></i>
                        </a>
                        <ul class="dropdown-menu dropdown-user">';
            foreach(EXPLENATIVES as $key => $button){
                $btns .= '<li><a href="#">'.$button.'</a></li>';
            }
            $btns .= '</ul>';
        }else{    
            $btns .= '<li><a href="#">'.$btn.'</a></li>';
        }
    }
    
    $btns .= '</ul>';
    return $btns;
}            

Love this new implementation!

?>

down

eparkerii at carolina dot rr dot com ¶

16 years ago

Found something interesting.  The following define:



<?php

define("THIS-IS-A-TEST","This is a test");

echo THIS-IS-A-TEST;

?>



Will return a '0'.



Whereas this:



<?php

define("THIS_IS_A_TEST","This is a test");

echo THIS_IS_A_TEST;

?>



Will return 'This is a test'.



This may be common knowledge but I only found out a few minutes ago.



[EDIT BY danbrown AT php DOT net: The original poster is referring to the hyphens versus underscores.  Hyphens do not work in defines or variables, which is expected behavior.]

down

chris at peeto dot net ¶

14 years ago

The value of a constant can be the value of another constant.

<?php

define("NEW_GOOD_NAME_CONSTANT", "I have a value");
define("OLD_BAD_NAME_CONSTANT", NEW_GOOD_NAME_CONSTANT);

echo NEW_GOOD_NAME_CONSTANT; // current
echo OLD_BAD_NAME_CONSTANT; // legacy

?>

down

Jamie ¶

13 years ago

This is obvious, but easy to forget: if you include a file, the include file can only make use of constants already defined. For example:



<?php

define("VEG","cabbage");

require("another file");

define("FRUIT","apple");



// "another file":

echo VEG; // cabbage

echo FRUIT; // FRUIT

?>

down

-2

swisschocolate at cmail dot nu ¶

8 years ago

I think worth mentioning is that define() appears to ignore invalid constant names.
One immediate implication of this seem to be that if you use an invalid constant name you have to use constant() to access it and obviously that you can't use the return value from define() to tell you whether the constant name used is invalid or not.

For example:
$name = '7(/!§%';
var_dump(define($name, "hello")); // outputs bool(true)
var_dump(constant($name)); // outputs string(5) "hello"

down

-3

axew3 at axew3 dot com ¶

8 years ago

Php 7 - Define: "Defines a named constant at runtime. In PHP 7, array values are also accepted."

But prior PHP 7, you can maybe do this, to pass an array elsewhere using define:

$to_define_array = serialize($array);
define( "DEFINEANARRAY", $to_define_array );

... and so ...

$serialized = DEFINEANARRAY; // passing directly the defined will not work
      $our_array = unserialize($serialized); 

print_r($our_array);

down

-3

Anonymous ¶

7 years ago

There's an undocumented side-effect of setting the third parameter to true (case-insensitive constants): these constants can actually be "redefined" as case-sensitive, unless it's all lowercase (which you shouldn't define anyway).

The fact is that case-sensitive constants are stored as is, while case-insensitive constants are stored in lowercase, internally. You're still allowed to define other constants with the same name but capitalized differently (except for all lowercase).

<?php
  // "echo CONST" prints 1, same as "echo const", "echo CoNst", etc.
  define('CONST', 1, true);
  echo CONST; // Prints 1

  define('CONST', 2);
  echo CONST; // Prints 2
  echo CoNsT; // Prints 1
  echo const; // Prints 1

  // ** PHP NOTICE: Constant const already defined ** 
  define('const', 3);
  echo const; // Prints 1
  echo CONST; // Prints 2
?>

Why would you use this?

A third party plugin might attempt to define a constant for which you already set a value. If it's fine for them to set the new value, assuming you cannot edit the plugin, you could define your constant case-insensitive. You can still access the original value, if needed, by using any capitalization other than the one the plugin uses. As a matter of fact, I can't think of another case where you would want a case-insensitive constant...

down

-5

me at mariusgerum dot de ¶

6 years ago

You can define constants with variable names (works also with constant values or variables or array values or class properties and so on - as long it's a valid constant name).

<?php
    
    # Define a constant and set a valid constant name as string value
    define("SOME_CONSTANT", "NEW_CONSTANT");
    
    # Define a second constant with dynamic name (the value from SOME_CONSTANT)
    define(SOME_CONSTANT, "Some value");
    
    # Output
    echo SOME_CONSTANT; // prints "NEW_CONSTANT"
    echo "<br>";
    echo NEW_CONSTANT; // prints "Some value"
    
    ?>

Needless to say that you'll lose your IDE support for refactoring and highlighting completely for such cases.
No clue why someone would / could actually use this but i thought it's worth mentioning.

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