ConFoo 2025

define

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

defineОпределяет именованную константу

Описание

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

Определяет именованную константу во время выполнения.

Список параметров

constant_name

Имя константы.

Замечание:

Возможно определить константы с помощью функции define() зарезервированными или даже некорректными именами, значения которых могут быть (только) получены через функцию constant(). Однако, делать это не рекомендуется.

value

Значение константы. В PHP 5 value должно быть скалярным значением (int, float, string, bool или null). В PHP 7 также возможно использовать тип array.

Внимание

Хотя возможно определить константы с типом resource, не рекомендуется это делать, поскольку может привести к непредсказуемому поведению.

case_insensitive

Если параметр установлен как true, то константа будет определена без учёта регистра. По умолчанию константа чувствительна к регистру, то есть CONSTANT и Constant представляют разные значения.

Внимание

Начиная с PHP 7.3.0, определение нечувствительных к регистру констант объявлено устаревшим. Начиная с PHP 8.0.0, допустимым значением является только false, передача true вызовет предупреждение.

Замечание:

Нечувствительные к регистру константы хранятся в нижнем регистре.

Возвращаемые значения

Функция возвращает true в случае успешного выполнения или false, если возникла ошибка.

Список изменений

Версия Описание
8.0.0 Передача true в case_insensitive теперь выдаёт ошибку уровня E_WARNING. Передача false всё ещё разрешена.
7.3.0 Параметр case_insensitive объявлен устаревшим и будет удалён в версии 8.0.0.
7.0.0 Допустимы значения типа array.

Примеры

Пример #1 Определение констант

<?php
define
("CONSTANT", "Hello world.");
echo
CONSTANT; // выводит "Hello world."
echo Constant; // выводит "Constant" и выдаёт уведомление.

define("GREETING", "Hello you.", true);
echo
GREETING; // выводит "Hello you."
echo Greeting; // выводит "Hello you."

// Начиная с PHP 7
define('ANIMALS', array(
'собака',
'кошка',
'птица'
));
echo
ANIMALS[1]; // выводит "кошка"

?>

Пример #2 Определение констант зарезервированными именами

Этот пример демонстрирует возможность определения константы с тем же именем, что и магическая константа. Поскольку это, очевидно, сбивающее с толку поведение, не рекомендуется делать это на практике.

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

Результат выполнения приведённого примера:

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

Смотрите также

  • defined() - Проверяет, существует ли константа с заданным именем
  • constant() - Возвращает значение константы
  • Смотрите раздел Константы

Добавить

Примечания пользователей 11 notes

up
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.
up
24
@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".)
up
30
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!');
?>
up
5
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!

?>
up
2
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.]
up
1
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

?>
up
0
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
?>
up
-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"
up
-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);
up
-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...
up
-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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