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
 
 if ( defined ( 'TEST' )) {
    echo  TEST ;
}
 ?>

参见

define() - 定义一个常量
constant() - 返回一个常量的值
get_defined_constants() - 返回所有常量的关联数组，键是常量名，值是常量值
function_exists() - 如果给定的函数已经被定义就返回 TRUE
Constants这一节

用户评论:

[#1] fernandopsilveira at yahoo dot com dot br [2015-09-23 17:05:06]

Just like define(), defined() will check for constants exactly as specified. So, if you want to check a constant in a namespace, you will need to specify the namespace in your call to defined(), even if you're calling defined() from within a namespace.

To check for a defined constant within current namespace, you can use __NAMESPACE__ magic variable:

<?php namespace mytest; define('mytest\CONSTANTINE', 'value'); if (defined(__NAMESPACE__ . '\CONSTANTINE')) { print "Constant is defined and is valued " . CONSTANTINE; } ?>

[#2] passerbyxp at gmail dot com [2012-11-02 12:10:29]

This function, along with constant(), is namespace sensitive. And it might help if you imagine them always running under the "root namespace":

<?php
namespace FOO\BAR
{
    const WMP="wmp";
    function test()
    {
        if(defined("WMP")) echo "direct: ".constant("WMP"); //doesn't work;
        elseif(defined("FOO\\BAR\\WMP")) echo "namespace: ".constant("FOO\\BAR\\WMP"); //works
        echo WMP; //works
    }
}
namespace
{
    \FOO\BAR\test();
}

[#3] Lars Lernestal [2011-10-25 02:03:25]

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

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

[#4] vindozo at gmail dot com [2010-08-31 02:21:01]

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.')); ?>

[#5] audvare at gmail dot com [2010-05-16 19:35:05]

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.

[#6] r dot hartung at roberthartung dot de [2009-12-03 23:45:03]

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()); ?>

[#7] info at daniel-marschall dot de [2009-07-11 13:51:36]

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".

[#8] tris+php at tfconsulting dot com dot au [2009-03-26 18:40:18]

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'))...

[#9] reachmike at hotpop dot com [2008-11-30 22:17:50]

You may find that if you use <?php= ?> 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:

<?php= 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:

<?php= 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.

[#10] daniel at neville dot tk [2008-07-14 10:48:49]

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.

[#11] Shaun H [2008-03-28 11:30:57]

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; ?>

[#12] Joel [2007-08-20 05:35:57]

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.

[#13] ndove at cox dot net [2005-01-27 19:20:03]

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

[#14] Craig at chatspike dot net [2003-11-30 11:57:56]

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 :)