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defined

(PHP 4, PHP 5)

defined — 检查某个名称的常量是否存在

说明

bool defined ( string $name )

检查该名称的常量是否已定义。

Note:

如果你要检查一个变量是否存在，请使用 isset()。 defined() 函数仅对 constants 有效。如果你要检测一个函数是否存在，使用 function_exists()。

参数

name

常量的名称。

返回值

如果该名称的常量已定义，返回 TRUE；未定义则返回 FALSE。

范例

Example #1 检查常量

<?php
/* Note the use of quotes, this is important.  This example is checking
 * if the string 'TEST' is the name of a constant named TEST */
if (defined('TEST')) {
    echo TEST;
}
?>

参见

杂项 函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 检查某个名称的常量是否存在

用户评论:

Lars Lernestal (25-Oct-2011 10:03)

if you want to check id a class constant is defined use self:: before the constant name:

<?php
defined('self::CONSTANT_NAME');
?>

vindozo at gmail dot com (31-Aug-2010 10:21)

If you wish to protect files from direct access I normally use this:

index.php:

<?php
// Main stuff here
define('START',microtime());

include "x.php";
?>

x.php:

<?php
defined('START')||(header("HTTP/1.1 403 Forbidden")&die('403.14 - Directory listing denied.'));
?>

audvare at gmail dot com (17-May-2010 03:35)

For compatibility with PHP 5.3 and 5.2 and dirname(__FILE__) vs __DIR__ constant:

<?php
$dir = NULL;
if (defined('__DIR__')) {
  $dir = __DIR__;
}
else {
  $dir = dirname(__FILE__);
}
?>

Not for every time; you should probably just define this elsewhere so every time the script needs the current directory just use $dir which was taken from that bit.

r dot hartung at roberthartung dot de (04-Dec-2009 07:45)

You can use the late static command "static::" withing defined as well. This example outputs - as expected - "int (2)"

<?php
  abstract class class1
  {
    public function getConst()
    {
      return defined('static::SOME_CONST') ? static::SOME_CONST : false;
    }
  }
 
  final class class2 extends class1
  {
    const SOME_CONST = 2;
  }
 
  $class2 = new class2;
 
  var_dump($class2->getConst());
?>

info at daniel-marschall dot de (11-Jul-2009 09:51)

I found something out: defined() becomes probably false if a reference gets lost.

<?php

session_start(); // $_SESSION created
define('SESSION_BACKUP', $_SESSION);
if (defined('SESSION_BACKUP')) echo 'A';
session_unset(); // $_SESSION destroyed
if (defined('SESSION_BACKUP')) echo 'B';

?>

You will see "A", but not "B".

tris+php at tfconsulting dot com dot au (27-Mar-2009 02:40)

Before using defined() have a look at the following benchmarks:

true                                       0.65ms
$true                                      0.69ms (1)
$config['true']                            0.87ms
TRUE_CONST                                 1.28ms (2)
true                                       0.65ms
defined('TRUE_CONST')                      2.06ms (3)
defined('UNDEF_CONST')                    12.34ms (4)
isset($config['def_key'])                  0.91ms (5)
isset($config['undef_key'])                0.79ms
isset($empty_hash[$good_key])              0.78ms
isset($small_hash[$good_key])              0.86ms
isset($big_hash[$good_key])                0.89ms
isset($small_hash[$bad_key])               0.78ms
isset($big_hash[$bad_key])                 0.80ms

PHP Version 5.2.6, Apache 2.0, Windows XP

Each statement was executed 1000 times and while a 12ms overhead on 1000 calls isn't going to have the end users tearing their hair out, it does throw up some interesting results when comparing to if(true):

1) if($true) was virtually identical
2) if(TRUE_CONST) was almost twice as slow - I guess that the substitution isn't done at compile time (I had to double check this one!)
3) defined() is 3 times slower if the constant exists
4) defined() is 19 TIMES SLOWER if the constant doesn't exist!
5) isset() is remarkably efficient regardless of what you throw at it (great news for anyone implementing array driven event systems - me!)

May want to avoid if(defined('DEBUG'))...

phrank (30-Jan-2009 09:17)

@ ndove at cox dot net

But you receive a Fatal error 'Undefined class constant' if the constant is not defined.:-)

reachmike at hotpop dot com (01-Dec-2008 06:17)

You may find that if you use <?= ?> to dump your constants, and they are not defined, depending on your error reporting level, you may not display an error and, instead, just show the name of the constant. For example:

<?= TEST ?>

...may say TEST instead of an empty string like you might expect. The fix is a function like this:

<?php

function C(&$constant) {
    $nPrev1 = error_reporting(E_ALL);
    $sPrev2 = ini_set('display_errors', '0');
    $sTest = defined($constant) ? 'defined' : 'not defined';
    $oTest = (object) error_get_last();
    error_reporting($nPrev1);
    ini_set('display_errors', $sPrev2);
    if ($oTest->message) {
        return '';
    } else {
        return $constant;
    }
}

?>

And so now you can do:

<?= C(TEST) ?>

If TEST was assigned with define(), then you'll receive the value. If not, then you'll receive an empty string.

Please post if you can do this in fewer lines of code or do something more optimal than toggling the error handler.

Anonymous (21-Oct-2008 04:36)

Nice one, here's may addition:

index.php:

<?php
// Main stuff here
define('SITE_IN', 1);
include "x.php";
?>

x.php:

<?php
if (!defined('SITE_IN') or !constant('SITE_IN')) die('Direct access not allowed!');
?>

A bit of extra protection, a bit paranoid yes, it's good to be :)

daniel at neville dot tk (14-Jul-2008 06:48)

My preferred way of checking if a constant is set, and if it isn't - setting it (could be used to set defaults in a file, where the user has already had the opportunity to set their own values in another.)

<?php

defined('CONSTANT') or define('CONSTANT', 'SomeDefaultValue');

?>

Dan.

admin at rune-city dot com (12-Jul-2008 12:39)

If you wish to protect files from direct access I normally use this:

index.php:

<?php
// Main stuff here
define('SITE_IN', 1);
include "x.php";
?>

x.php:

<?php
if (!defined('SITE_IN')) die('Direct access not allowed!');
?>

Shaun H (28-Mar-2008 07:30)

I saw that PHP doesn't have an enum function so I created my own. It's not necessary, but can come in handy from time to time.

<?php
    function enum()
    {
        $args = func_get_args();
        foreach($args as $key=>$arg)
        {
            if(defined($arg))
            {
                 die('Redefinition of defined constant ' . $arg);
            }

            define($arg, $key);
        }
    }
   
    enum('ONE','TWO','THREE');
    echo ONE, ' ', TWO, ' ', THREE;
?>

Joel (20-Aug-2007 01:35)

If your constants don't show up in your included or required files, then you probably have php safe mode turned on!

I ran into this problem, I forgot to turn of safe mode when I was creating a new site.

ndove at cox dot net (28-Jan-2005 03:20)

In PHP5, you can actually use defined() to see if an object constant has been defined, like so:

<?php

class Generic
{
    const WhatAmI = 'Generic';
}

if (defined('Generic::WhatAmI'))
{
    echo Generic::WhatAmI;
}

?>

Thought it may be useful to note.

-Nick

Craig at chatspike dot net (30-Nov-2003 07:57)

This can be useful if you want to protect pages which get included from outsiders eyes, on your mail page (the page viewable by people) put define("X", null); then on all your other pages, you can then do something like:

<?php
if (!defined("X")) {
    echo "You Cannot Access This Script Directly, Have a Nice Day.";
    exit();
}
?>

And your page is a good as protected :)