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define

(PHP 4, PHP 5)

define — 定义一个常量

说明

bool define ( string $name , mixed $value [, bool $case_insensitive = false ] )

在运行时定义一个常量。

参数

name

常量名。

value

常量的值；仅允许标量和 null。标量的类型是 integer， float，string 或者 boolean。 也能够定义常量值的类型为 resource ，但并不推荐这么做，可能会导致未知状况的发生。

case_insensitive

如果设置为 TRUE，该常量则大小写不敏感。默认是大小写敏感的。比如， CONSTANT 和 Constant 代表了不同的值。

Note:

大小写不敏感的常量以小写的方式储存。

返回值

成功时返回 TRUE， 或者在失败时返回 FALSE.

范例

Example #1 定义常量

<?php
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."

?>

参见

杂项 函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 定义一个常量

用户评论:

kriscraig at php dot net (24-Mar-2012 05:47)

Some people like to have an ambiguous, engine-agnostic database include specified by assigning a single config variable to one of a series of constants.  Unfortunately, this can easily become needlessly clunky if these defines are handled in an included config.php file, since more than one hook will throw an ugly "already defined" error.

Here's a simple way to accomplish this architectural model without having to create a bunch of clumsy sanity checks that compromise scalability:

<?php

/* Available databases.  --Kris */
$dbengines = array();
$dbengines[] = "mysql";
$dbengines[] = "mysqli";
$dbengines[] = "pgsql";

foreach ( $dbengines as $engine )
{
    if ( !defined( "_" . strtoupper( $engine ) . "_" ) )
    {
        define( "_" . strtoupper( $engine ) . "_", $engine );
    }
}

/* Which database engine shall we use?  --Kris */
$sql_type = _MYSQLI_;

?>
Then, somewhere later in the stack....

<?php

/* Include the file with the desired sql class.  --Kris */
require_once( $sql_type . ".php" );

/* Now you can use the same code for any supported db engine.  --Kris */
$sql = new sql();
$res = $sql->query( .... );

david at rayninfo dot co dot uk (15-Mar-2012 07:42)

define('DB_NAME','php-constants');
$DB_NAME=DB_NAME;
echo "\n$DB_NAME"; // define value now in a variable
echo "\n".DB_NAME; // must never quote a define

webmaster@PersonalInformationTechnology. (26-Oct-2011 11:35)

Be careful when testing for defined boolean values.
For example, consider a section of code defining other constants, and  one control to see if they are defined.
file constants.php
<?php
  define("FOO", value);
  define("BAR", value2);
  define("CONST_DEFINED", true);
?>

and, in another file
<?php
  if(CONST_DEFINED)
  {/* use them  */}
?>

This will always return true, regardless if the constants.php file was successfully included (at least in 5.2.17)

A better approach is to define the negative:
  define("CONST_NOT_DEF", false)
and test that negative
  if(!CONST_NOT_DEF)....

Checking for defined("CONST_DEFINED") also works, but why have a boolean constant if one isn't going to use the value directly?

phen at adnerdum dot org (06-Jul-2011 09:32)

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'

kobrasrealm at gmail dot com (11-Jun-2011 02:31)

When going back to rewrite code, it might be tempting to have a constant &reference a variable, but it doesn't work.

<?php
header("Content-type: text/plain");
$variable = "value";
define("MYCONSTANT", &$variable);
  echo $variable."\n";
  echo MYCONSTANT."\n";
  echo "------\n";
$variable = "different";
  echo $variable."\n";
  echo MYCONSTANT;
?>

Produces this:

value
value
------
different
value

I doubt it's supposed to work, either. :)

Jamie (01-Mar-2011 09:24)

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
?>

andreas at cap-systems dot com (05-Feb-2011 12:54)

Constant name must not written as string. The following code works also:

<?php
define(MY_NAME, 'value');
define('ANOTHER_NAME', 'value2');

if(defined('MY_NAME')) {
  echo MY_NAME . '<br />';
}

if(defined('ANOTHER_NAME')) {
  echo ANOTHER_NAME . '<br />';
}

echo MY_NAME . '<br />';
echo ANOTHER_NAME . '<br />';
?>

Output:
value
value2
value
value2

Tested with:
- PHP 5.3.1 on XAMPP on Windows Vista (local)
- PHP 5.2.42 on Apache 2.0 on Linux (remote)
- PHP 5.2.12 on Apache 2.0 on Linux (remote)

angelos at unkreativ dot org (09-Jan-2011 09:18)

Something I found out the "hard" way today: integers starting with 0 won't work.
<?php
define('SOMECONSTANT', 09900);
var_dump(SOMECONSTANT);
?>
Produces int(0)

<?php
define('SOMECONSTANT', 9900);
var_dump(SOMECONSTANT);
?>
Produces int(9900)

toph at farmer corp dot c om (16-Oct-2010 10:51)

Hi,

Here is a trick to use constants within the heredoc notation.

<?php
define('MY_CONSTANT','foo bar')
$cst = 'cst';
function cst($constant){
    return $constant;
}

$string = <<<EOF
My constante MY_CONSTANT : {$cst(MY_CONSTANT)} ...
EOF;
echo $string;
?>

That will echo :

My constante MY_CONSTANT : foo bar ...

As you can use function within the heredoc notation the idea is to use a fonction to return the value of the constant.
Be carefull, to use a function, it's necessary to declare the name of the function as a variable and to use that varaible during the "call" of the constant.

It works for me and doesn't seem too messy.

Hope that will help.

rush at bigup dot com dot pa (10-Aug-2010 06:23)

Just in case:

You cannot set a constant to be a class.  For example:
define ("WSDLCLIENT",new soapclient(WSDLSERVER, 'wsdl'));

You cannot do this.

chris at peeto dot net (26-Mar-2010 03:15)

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

?>

bohwaz.net (16-Aug-2009 01:54)

Note that define don't care at all about the constant name, you can give everything as a constant name, even if the PHP documentation says that the allowed characters are [a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*

Just try this :

<?php define('anything::()$', 'test'); ?>

You won't be able to access this constant though, it will throw a parse error, either through constant() or define(), but if you do that :

<?php print_r(get_defined_constants()); ?>

You will see that the constant is registered at the end of the array. Yes that's totally useless. But you can try other things that will work with constant() :

<?php define('...', 'test'); echo constant('...'); ?>

Will echo 'test'. Yeah that's a pretty strange behavior.

And please note that defining object constants outside of objets don't work :

<?php
class myObject { }
define('myObject::CONSTANT', 'test');
?>

Anonymous (20-Jul-2009 06:31)

My notice to: redeclare define -  it is possible

php -v

--> PHP 5.2.6-3ubuntu4.1 with Suhosin-Patch 0.9.6.2
Zend Engine v2.2.0, Copyright (c) 1998-2008 Zend Technologies
php -r "define('TEST','foo',true);
var_dump(TEST);
define('TEST','bar');
var_dump(TEST);"

string(3) "foo"
string(3) "bar"

greetings

sixd at php dot net (17-Jul-2009 10:24)

A note on redefining:

Constants can't be redefined:

$ php -r "define('A', 1); var_dump('A'); define('A', 2); var_dump('A');";
string(1) "A"
PHP Notice:  Constant A already defined in Command line code on line 1
string(1) "A"

But using the case insensitive setting shows that the internal
representation is lower case:

$ php -r "define('A', 1, true); var_dump('A'); define('A', 2); var_dump('A');";
string(1) "A"
string(1) "A"

Note the lower case 'a' in the second define() here:
$ php -r "define('A', 1, true); var_dump('A'); define('a', 2); var_dump('A');";
string(1) "A"
PHP Notice:  Constant a already defined in Command line code on line 1
string(1) "A"

danbettles at yahoo dot co dot uk (14-Apr-2009 07:56)

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!');
?>

Aldonio (27-Mar-2009 04:24)

This code will define every key of an array, and if its a multidimentional array it will define it as parent_child1_child2_etc.

<?php
function define_array( $array, $keys = NULL )
{
    foreach( $array as $key => $value )
    {
        $keyname = ($keys ? $keys . "_" : "") . $key;
        if( is_array( $array[$key] ) )
            define_array( $array[$key], $keyname );
        else
            define( $keyname, $value );
    }
}
?>

ceo at l-i-e dot com (11-Mar-2009 10:04)

In PHP 4.4.9, you cannot use certain names for constants.

Specifically, reserved words such as 'PRINT', 'IF', etc.

Oddly enough, you get different behaviour for some:

<?php
define('IF', 42);
echo "IF: ", IF, "<hr />\n";
?>

yields PHP Parse error:  syntax error, unexpected T_IF

<?php
define('PRINT', 13);
echo "PRINT: ", PRINT, "<hr />\n";
?>

yields PHP Parse error:  syntax error, unexpected ')'

Not sure if this is still true in PHP 5 or not, but it's worth noting if it is.

Anonymous (09-Mar-2009 09:03)

With php 5.30 it is possible to make some "restrictions", in case u use namespaces...
The following function allows only uppercase and, if 3 param isset, gives E_USER_ERROR instead of E_NOTICE:

<?php
namespace bla;

function define($c,$v,$x=false) {
    $c = strtoupper($c);
    if (!defined($c)) {
        // not to use \define here, as the syntax highlighting does not work correctly here...
        return define($c, $v, true);
    } else if ($x == true) {
        trigger_error('setting constant "'.$c.'" is not allowed', E_USER_ERROR);
    }
    return null;
}

define('PHP_VERSION', '1');
define('PHP_VERSION', '1') || trigger_error('constant already defined', E_USER_NOTICE);
define('PHP_VERSION', '1', true);
?>

jv at vip dot ie (09-Mar-2009 12:14)

Probably the most efficient way of creating a constant array is to put it in a function like this:

<?php
function my_const_arr()
{
   return array
   (
      'key1' => 'val 1',
      'key2' => 'va' 2'
   );
}
?>

Then use at will like so...
<?php $my_const_arr = my_const_arr(); ?>

usuroer dot king at hotmail dot co dot uk (08-Mar-2009 12:15)

Referring to "This-Is-A-Test" constant below.

When echoing it the script thinks you performing a series of minus (subtraction) thus since THIS and IS, A, TEST are not defined they are 0 by default. So 0-0-0-0 = 0.

As explained that a hyphen cannot be used within a variable.

downwind at web dot de (02-Feb-2009 04:48)

I'm reffering to the note below by mittiprovence.

The described behavior has nothing to do with the fact that there is a constant in the condition.

Comparing a non-numeric string to the integer 0 by using == will return true, since the string will be casted to integer - which will be zero. If your string starts with a number, it will be castet to an integer with the number as value.

To avoid the described behavior, you should use === instead of ==.

Finally, the behavior discribed by mittiprovence is exacly the expected behavior as defined in the manual.

For further information read the manual:

http://www.php.net/manual/en/types.comparisons.php

http://www.php.net/manual/en/language.operators.comparison.php

http://www.php.net/manual/en/language.types.string.php #language.types.string.conversion

<?php

if (0 == "some string") { // is true
echo "0 == 'some string'";
}

if (1 == "some string") { // is false
echo "1 == 'some string'";
}

if (1 == "1 some string") { // is true
echo "1 == '1 some string'";
}

if (0 === "some string") { // is false
echo "0 === 'some string'";
}

?>

eparkerii at carolina dot rr dot com (22-Oct-2008 08:01)

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.]

pistachio (02-Sep-2008 12:41)

A better solution to that of thscheer at web dot de, is to simply serialize the array.

for example:

<?php

$fruits = array("apple", "banana", "orange");

define("FRUITS", serialize($fruits));

// Now to use it
$my_fruits = unserialize(FRUITS);

?>

As you can see this is a much simpler and more efficient way to stuff arrays into constants.

jan at webfontein dot nl (27-Jul-2008 01:20)

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 )
?>

thscheer at web dot de (20-Jun-2008 10:18)

How to define an array as constant with define?

there is a simple way to define an array of values as constant...

just

define ("constant-name","value1,value2,value3,value4");

$t_array = preg_split('/,/', constant(constant-name), -1, PREG_SPLIT_NO_EMPTY);

var_dump($t_array);

array(4) { [0]=> string(6) "value1" [1]=> string(6) "value2" [2]=> string(6) "value3" [3]=> string(6) "value4" }

Now you can iterate over the array defined as constant ...

ceo at l-i-e dot com (05-May-2008 04:30)

I'm not sure if it's version dependent, but apparently this works:
define('APPLICATION_NOW', time());

This can be quite handy for time-dependent code...

You can then do:
class foo {
  $private now = APPLICATION_NOW;
}

and have a consistent time-stamp across the application, which is nice.

Works for me in 5.2.5

I didn't think it worked before...

jason@sitehatchery dot com (25-Apr-2008 07:28)

You can store your constant as the value of a variable like this:

$variable=VARIABLE;

To access the constant, do this:  constant($variable).

Beware: if your variable changes, then you loose your reference to the constant - but the constant itself will not change.

I've found this especially useful for storing the constant string   (i.e. VARIABLE) in a database.

rayro at gmx dot de (15-Sep-2007 10:53)

just a note to the previous post by e s lin d sey at g mail dot co m:

Well as u said, it is not the best workaround. To resolve this "Problem" without getting incredible loss of performance, you are able to make the use of "variable functions" like "variable variables":

<?php
$def = 'constant';
$string = <<<END
    This is PHP running on<br />
    {$def('PHP_OS')}
END;
var_dump($string);
?>

In this example we make use of the "constant" function which is builtin, and it will output:

string(37) " This is PHP running on
WINNT"

Tested on PHP 5...
Best regards

e s lin d sey at g mail dot co m (13-Aug-2007 09:38)

Constants can't be referenced from within quotes or (more importantly IMHO) from within HEREDOC syntax. This is a huge drawback in my opinion. Here's two of my workarounds:

<?php

//PHP5 only -- more efficient
$constarray = get_defined_constants(true);
foreach($constarray['user'] as $key => $val)
    eval(sprintf('$_CONSTANTS[\'%s\'] = \'%s\';', addslashes($key), addslashes($val)));

//PHP4+ -- less efficient because it defines (potentially) hundreds of unnecessary constants
foreach(get_defined_constants() as $key => $val)
    eval(sprintf('$_CONSTANTS[\'%s\'] = \'%s\';', addslashes($key), addslashes($val)));

?>

Now you can refer to your defined constants using the $_CONSTANTS array. Note that because this is NOT a superglobal, a few caveats apply:

<?php

//run code snippet here to define $_CONSTANTS in global scope...

$mv = $_CONSTANTS['FOO']; //works

function my_function1()
{
    $mv = $_CONSTANTS['BAR']; //doesn't work! not defined!
}

function my_function2()
{
    global $_CONSTANTS;
    $mv = $_CONSTANTS['BAR']; //ah, this works!
}

?>

I realize this is not ideal, either for performance or for convenience of being able to refer to constants without regard to scope, but it is a workaround that works. Depending on your application, it may be easier to shift your paradigm a bit and use the following method instead, declaring your constants as variables first:

<?php

//first, define our constants...
$_CONSTANTS['FOO'] = 'bar';
$_CONSTANTS['BAR'] = 'foo';

//now, turn them into real constants...
foreach($_CONSTANTS as $key => $val)
    define($key, $val);

//now, you can use either format to refer to a constant
print($_CONSTANTS['FOO']);   //prints 'bar'
print(FOO);                  //prints 'bar'
print("$_CONSTANTS['FOO']"); //prints 'bar'
                             //prints 'blah bar blah'
print <<<EOF
  blah {$_CONSTANTS['FOO']} blah
EOF;

?>

Hope this helps some of you out there, I know being able to utilize my constants in HEREDOC helped me a ton!

smifffy at smith-net dot org dot uk (26-Jul-2007 12:23)

a basic function to auto assign a numeric value that increase by itself each time - used in some permission style scripts

<?php
function define_bitwise($constant_name, $reset = False)
{
    static $bitwise_next = 1;

    if ( $reset === True )
    {
        $bitwise_next = 1;
    }

    define($constant_name, $bitwise_next);
       
    $bitwise_next += $bitwise_next;
}
?>

when reset it set to True, resets that value to 1 and starts afresh

creeves at dja dot com (19-Jul-2007 02:59)

Intersting to note:

I found this in php5 on windows

If you try to unset a constant (which you cant but maybe you have a Jr. programmer or something).  The error message that is thrown is:

<?php
define ("SOME_CONSTANT", true);
unset (SOME_CONSTANT);
?>

Parse error: parse error, unexpected ')', expecting T_PAAMAYIM_NEKUDOTAYIM

nl-x at bita dot nl (09-Jul-2007 06:34)

Watch out. You can define a new constant with the name NULL with define("NULL","FOO");. But you must use the function constant("NULL"); to get it's value. NULL without the function call to the constant() function will still retrieve the special type NULL value.
Within a class there is no problem, as const NULL="Foo"; will be accessible as myClass::NULL.

stangelanda at arrowquick dot com (01-Jan-2007 02:59)

An improvement on the function from bobbykjack at yahoo dot co dot uk on the concept by richard dot quadling at bandvulc dot co dot uk:
<?php
function adefine($constant_name, $value=null) {
    static $increment = 0; // 1 for bitmask

    if (is_null($value)) {
        define($constant_name, ++$increment); // $increment=$increment<<1 for bitmask
    } else {
        define($constant_name, $value);
        if (is_numeric($value)) {
            $increment = $value;
        }
    }
}
?>
If you pass it a second argument it defines it normally, and resets the increment if the value is numeric.  This way the function can replace define, and you can reset the counter for a new set of constants.
<?php
adefine ('RULE_CALLBACK_FORMAT', 1); // 1
adefine ('RULE_CHANGE_CALLBACK_ON_ERROR'); // 2
adefine ('RULE_CHANGE_COMPARE_DATE'); // 3
adefine('KEYWORD', 'hodgepodge'); // hodgepodge <-- defined normally
adefine ('RULE_CHANGE_ON_DATE'); // 4

adefine ('ERROR_DESC', 1); // 1 <-- Counter reset
adefine ('ERROR_EXPECTED_RESULT'); // 2
?>

(12-Feb-2005 04:45)

Better pack with define() for all who really miss Java package management:

Use this "manifest.php" on very first script start or copy it to your config.somehow.php.

<?php
$__packages = array(
    "org.ilove.java.more",
    "org.ilove.python.too",
    "net.php.isok"
);

define("C_IS_WINDOWS", false);
define("C_DIR", (C_IS_WINDOWS ? "\\" : "/"));
define("C_PATH_ROOT", str_replace("/", C_DIR, $_SERVER["DOCUMENT_ROOT"]).C_DIR);
define("C_PATH_CORE", C_PATH_ROOT."core".C_DIR);
define("C_PATH_CLASS", C_PATH_CORE."classes".C_DIR);
define("C_APPLICATION_BASE", C_PATH_CORE.C_DIR."application".C_DIR);

$total_packages = 0;
$i = sizeof($__packages);
while($i-- > 0) {
    $tokens = explode(".", $__packages[$i]);
    $j = sizeof($tokens);
    while($j-- > 0) {
        $token = strtolower(trim($tokens[$j]));
        if(strlen($token) > 0 && !defined($token)) {
            define($token, ($j == 0 ? C_PATH_CLASS : "").$tokens[$j].C_DIR);
            $total_packages++;
        }
    }
}
define("C_PACKAGE_COUNT", $total_packages);
?>

With restrictions on non-package constants, you now can call your files like that:

<?php

require_once org.ilove.java.more."Than.php";

?>

Regards
Robi

phpnet at trenkner dot de (14-Mar-2003 11:59)

---[Editor's Note]---
As of PHP 5.0.0 this is possible. You can define class-only constants, which can be called like Foo::Constant1 from the outside
---[End Note]---

Please keep in mind that

class AClass {
  define ("Const1", "Value1");
  ... }

didn't work. You have to make all your constant definitions before you open the class. So

define ("Const1", "Value1");
class AClass {
  ... }

would be correct.

radovan dot biciste at managestar dot com (07-Nov-2001 04:45)

Wonder how to work with variable which name is stored in a constant?
Here it is:
<?php
define("VAR_NAME","test");
// assigning value
${VAR_NAME} = "value";
// getting value back
echo ${VAR_NAME};
?>

ste at opk dot no (29-Aug-2001 06:41)

To use a constant to show an element of an array inside a string:
define ('C', 0); print ("element 0: {$a[C]}");
The { & } around the variable signals that what's inside should be treated as a variable and not a string.
Note that 'print ("a constant:{C}");' wont work as ZERO is a constant.