Hướng dẫn define php

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

define — Defines a named constant

Description

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

Parameters

The name of the constant.

Note:

It is possible to define() constants with reserved or even invalid names, whose value can (only) be retrieved with constant(). However, doing so is not recommended.

The value of the constant. In PHP 5, value must be a scalar value (int, float, string, bool, or null). In PHP 7, array values are also accepted.

Warning

While it is possible to define resource constants, it is not recommended and may cause unpredictable behavior.

If set to true, the constant will be defined case-insensitive. The default behavior is case-sensitive; i.e. CONSTANT and Constant represent different values.

Warning

Defining case-insensitive constants is deprecated as of PHP 7.3.0. As of PHP 8.0.0, only false is an acceptable value, passing true will produce a warning.

Note:

Case-insensitive constants are stored as lower-case.

Return Values

Returns true on success or false on failure.

Changelog

Examples

Example #1 Defining Constants

define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
echo Constant; // outputs "Constant" and issues a notice.define("GREETING", "Hello you.", true);
echo GREETING; // outputs "Hello you."
echo Greeting; // outputs "Hello you."

// Works as of PHP 7

Example #2 Constants with Reserved Names

This example illustrates the possibility to define a constant with the same name as a magic constant. Since the resulting behavior is obviously confusing, it is not recommended to do this in practise, though.

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

The above example will output:

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

See Also

• defined() - Checks whether a given named constant exists
• constant() - Returns the value of a constant
• The section on Constants

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:

define('ENABLE_UPLOADS', true);
?>

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

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

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

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.

@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".)

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").

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

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

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

Dale Landry ¶

5 years ago

With php 7 you can now define arrays.

consider the following code:


define

eparkerii at carolina dot rr dot com ¶

13 years ago

Found something interesting.  The following define:

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

Will return a '0'.

Whereas this:

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.]

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:

define("VEG","cabbage");
require("another file");
define("FRUIT","apple"); // "another file":
echo VEG; // cabbage
echo FRUIT; // FRUIT
?>

chris at peeto dot net ¶

12 years ago

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

define

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).

# 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 "
";
    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.

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"

axew3 at axew3 dot com ¶

6 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);

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).

  // "echo CONST" prints 1, same as "echo const", "echo CoNst", etc.
  define('CONST', 1, true);
  echo CONST; // Prints 1define('CONST', 2);
  echo CONST; // Prints 2
  echo CoNsT; // Prints 1
  echo const; // Prints 1

  // ** PHP NOTICE: Constant const already defined **

phen at adnerdum dot org ¶

11 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:

define('GOOD_OCTAL', 0700);
define('BAD_OCTAL', 0800);

print

jan at webfontein dot nl ¶

14 years ago

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

Example :

define('PAYMENT_IDEAL',"iDEAL Payment ( NL only )");
define('PAYMENT_MASTERCARD',"Mastercard Payment ( international )");$payparam='MASTERCARD';

echo

David Spector ¶

3 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".

agneady at gmail dot nospam dot please dot com ¶

11 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:

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

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.

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