PHP - Manual: define

2024-12-22

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define

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

define — 定义一个常量

说明

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

在运行时定义一个常量。

参数

constant_name: 常量名。

注意:
可以用 define() 定义保留关键词甚至无效名称的常量，它的值可以（仅可以）通过 constant() 获取。不过，不推荐这么做。
value: 常量的值。在 PHP 5 中，value 必须是 scalar 值（int、 float、string、boolean 或 null）。在 PHP 7 中， array 值也被允许。

警告
常量还可以定义为 resource 类型，但并不推荐这样做，因为可能会有不可预知的行为发生。
case_insensitive: 如果设置为 true，则该常量不区分大小写。默认是区分大小写的。比如， CONSTANT 和 Constant 代表了不同的值。

警告
PHP 7.3.0 起废弃了定义不区分大小写的常量。

注意:
不区分大小写的常量以小写的形式储存。

返回值

成功时返回 true，或者在失败时返回 false。

更新日志

版本	说明
7.3.0	废弃了 `case_insensitive`，并将在 8.0.0 版中移除。
7.0.0	允许 array 的值。

范例

示例 #1 定义常量


<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // 输出 "Hello world."
echo Constant; // 输出 "Constant" 并导致 Notice

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

//  PHP 7 起就可以运行了
define('ANIMALS', array(
    'dog',
    'cat',
    'bird'
));
echo ANIMALS[1]; // 输出 "cat"

?>

示例 #2 以保留名称定义常量

本例子说明了以魔术常量相同名称定义常量的能力。由于行为结果过于令人迷惑，所以实践中不推荐。


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

以上例程会输出：

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

参见

defined() - 检查某个名称的常量是否存在
constant() - 返回一个常量的值
关于常量的章节

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User Contributed Notes 16 notes

down

100

ravenswd at gmail dot com ¶

7 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 ¶

5 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".)

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danbettles at yahoo dot co dot uk ¶

13 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

me at mariusgerum dot de ¶

3 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.

down

chris at peeto dot net ¶

12 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

Dale Landry ¶

5 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 ¶

13 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

swisschocolate at cmail dot nu ¶

6 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

Anonymous ¶

5 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

Jamie ¶

11 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

axew3 at axew3 dot com ¶

5 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

-2

phen at adnerdum dot org ¶

10 years ago


To clear up a few thing:
Integers with 0 in front work. But since PHP (and many other languages) handle them as octal values, they're only allowed a range of 0-7:

<?php
 define('GOOD_OCTAL', 0700);
define('BAD_OCTAL', 0800);

 print GOOD_OCTAL;
 print '<br>';
 print BAD_OCTAL;
?>

Result:
448
0

---------------------------------------

Furthermore, 

writing the constant name without the quotation-marks (as mentioned in the notes) throws an E_NOTICE and should be avoided!

<?php
 define(TEST, 'Throws an E_NOTICE');
?>

Result:
Notice: Use of undefined constant TEST - assumed 'TEST'

down

-3

jan at webfontein dot nl ¶

13 years ago


For translating with variables and define, take also a look on the constant() function.

Example : 

<?php 
define('PAYMENT_IDEAL',"iDEAL Payment ( NL only )");
define('PAYMENT_MASTERCARD',"Mastercard Payment ( international )");

$payparam='MASTERCARD';

echo constant("PAYMENT_$payparam");

// output : 
// Mastercard Payment ( international )
?>

down

-2

David Spector ¶

2 years ago


It may be worth stating that a define function must be executed before its global constant is referenced.

Example:

Abc();
define("TEST", 23);
function Abc()
    {
    echo TEST;
    } // Abc

This code fails with a Notice-level message. TEST is treated here as being the string "TEST".

down

-3

agneady at gmail dot nospam dot please dot com ¶

8 months ago


If you happen to name your constant the same as a function name (either a built-in function or a user-defined one), PHP can handle this correctly based on context. For example:

<?php
function myfunc() {
   return 'function output';
}

define('MYFUNC', 'constant value');

// note that function names are NOT case-sensitive
// so calling MYFUNC() is the same as calling myfunc()

echo 'MYFUNC(): ' . MYFUNC() . ', MYFUNC: ' . MYFUNC;
?>

Output:
MYFUNC(): function output, MYFUNC: constant value

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-7

ohcc at 163 dot com ¶

2 years ago


A namespace constant can be defined using the define function, the constant defined this way is not global.

<?php
    namespace WuXiancheng;
    \define('China\Sichuan\Guangan\Yuechi\ZIP', 638300);
    echo ZIP; //Use of undefined constant ZIP
    echo \China\Sichuan\Guangan\Yuechi\ZIP; // 638300
?>

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官方地址：https://www.php.net/manual/en/function.define.php

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define

说明

参数

返回值

更新日志

范例

参见

User Contributed Notes 16 notes