constant

(PHP 4 >= 4.0.4, PHP 5)

constant — 返回一个常量的值

说明

mixed constant ( string $name )

通过 name 返回常量的值。

当你不知道常量名，却需要获取常量的值时， constant() 就很有用了。也就是常量名储存在一个变量里，或者由函数返回常量名。

该函数也适用 class constants。

参数

name: 常量名。

返回值

返回常量的值。如果常量未定义则返回 NULL 。

错误／异常

如果常量未定义，会产生一个 E_WARNING 级别的错误。

范例

Example #1 constant() 的例子

  <?php

define ( "MAXSIZE" ,  100 );

echo  MAXSIZE ;
echo  constant ( "MAXSIZE" );  // same thing as the previous line


 interface  bar  {
    const  test  =  'foobar!' ;
}

class  foo  {
    const  test  =  'foobar!' ;
}

 $const  =  'test' ;

 var_dump ( constant ( 'bar::' .  $const ));  // string(7) "foobar!"
 var_dump ( constant ( 'foo::' .  $const ));  // string(7) "foobar!"

 ?>

参见

define() - 定义一个常量
defined() - 检查某个名称的常量是否存在
Constants 这一节

用户评论:

[#1] t dot kmieliauskas at gmail dot com [2015-04-21 14:02:40]

If you are referencing class constant (either using namespaces or not, because one day you may want to start using them), you'll have the least headaches when doing it like this:

<?php class Foo { const BAR = 42; } ?>
<?php namespace Baz; use \Foo as F; echo constant(F::class.'::BAR'); ?>

since F::class will be dereferenced to whatever namespace shortcuts you are using (and those are way easier to refactor for IDE than just plain strings with hardcoded namespaces in string literals)

[#2] Richard J. Turner [2013-01-31 11:57:44]

As of PHP 5.4.6 constant() pays no attention to any namespace aliases that might be defined in the file in which it's used. I.e. constant() always behaves as if it is called from the global namespace. This means that the following will not work:

<?php class Foo { const BAR = 42; } ?>

<?php namespace Baz; use \Foo as F; echo constant('F::BAR'); ?>

However, calling constant('Foo::BAR') will work as expected.

[#3] adam at adamhahn dot com [2011-09-28 12:59:29]

This function is namespace sensitive when calling class constants.

Using:
<?php namespace sub; class foo { const BAR = 'Hello World'; } constant('foo::BAR'); // Error constant('sub\foo::BAR'); // works ?>

This does not seem to affect constants defined with the 'define' function. Those all end up defined in the root namespace unless another namespace is implicitly defined in the string name of the constant.

[#4] bohwaz [2010-07-05 06:28:59]

Return constants from an object. You can filter by regexp or match by value to find a constant name from the value.

Pretty useful sometimes.

<?php function findConstantsFromObject($object, $filter = null, $find_value = null) { $reflect = new ReflectionClass($object); $constants = $reflect->getConstants(); foreach ($constants as $name => $value) { if (!is_null($filter) && !preg_match($filter, $name)) { unset($constants[$name]); continue; } if (!is_null($find_value) && $value != $find_value) { unset($constants[$name]); continue; } } return $constants; } ?>

Examples :

<?php class Example { const GENDER_UNKNOW = 0; const GENDER_FEMALE = 1; const GENDER_MALE = 2; const USER_OFFLINE = false; const USER_ONLINE = true; } $all = findConstantsFromObject('Example'); $genders = findConstantsFromObject('Example', '/^GENDER_/'); $my_gender = 1; $gender_name = findConstantsFromObject('Example', '/^GENDER_/', $my_gender); if (isset($gender_name[0])) { $gender_name = str_replace('GENDER_', '', key($gender_name)); } else { $gender_name = 'WTF!'; } ?>

[#5] hellekin [2010-05-25 08:07:43]

Checking if a constant is empty is bork...

You cannot

<?php define('A', ''); define('B', 'B'); if (empty(B)) // syntax error if (empty(constant('B'))) // fatal error // so instead, thanks to LawnGnome on IRC, you can cast the constants to boolean (empty string is false) if (((boolean) A) && ((boolean) B)) // do stuff ?>

[#6] dachnik [2010-04-09 23:41:55]

You can define values in your config file using the names of your defined constants, e.g.

in your php code:
define("MY_CONST",999);

in you config file:
my = MY_CONST

When reading the file do this:

$my = constant($value); // where $value is the string "MY_CONST"

now $my holds the value of 999

[#7] roller [2009-06-21 08:21:34]

howto echo CONSTANT_NAME without warnings and "if " checking:

<?php !defined("CONSTANT_NAME") || constant("CONSTANT_NAME"); ?>

[#8] cory dot mawhorter @ ephective dot com [2008-09-08 14:08:48]

This is how I check to see if a bool constant is true:

<?php function consttrue($const) { return !defined($const) ? false : constant($const); } ?>

Examples
<?php var_dump(consttrue('UNDEFINED_CONST')); define('SOME_CONST', true); var_dump(consttrue('SOME_CONST')); define('SOME_CONST2', false); var_dump(consttrue('SOME_CONST2')); ?>

Returns
bool(false)
bool(true)
bool(false)

If it isn't defined it will return false, otherwise it will return the value of the constant... which would be either true/false depending on what you set it to.

[#9] [2007-02-01 20:29:30]

@XC:

That isn't necessary. If a constant is undefined, constant() returns NULL; simply suppressing the warning should be enough:

<?php if(defined('FOO') && constant('FOO') === 'bar'){ // ... } ?>

becomes

<?php if(@constant('FOO') === 'bar') { // ... } ?>

Note that in the first snippet, the call to constant isn't unnecessary as well, and adds a bit of overhead. If you're set on using the first notation, the following is better:

<?php if(defined('FOO') && FOO === 'bar') { // ... } ?>

[#10] XC [2007-01-19 02:13:03]

When you often write lines like

<?php if(defined('FOO') && constant('FOO') === 'bar') { ... } ?>

to prevent errors, you can use the following function to get the value of a constant.

<?php function getconst($const) { return (defined($const)) ? constant($const) : null; } ?>

Finally you can check the value with

<?php if(getconst('FOO') === 'bar') { ... } ?>

It's simply shorter.

[#11] narada dot sage at googlemail dot com [2006-07-13 06:01:17]

To access the value of a class constant use the following technique.

<?php class a { const b = 'c'; } echo constant('a::b'); // output: c ?>

[#12] service at dual-creators dot de [2006-05-16 10:00:57]

It's easily to user constant() and define() to translate some words from your database-saves.

For example:
You have a table userprofil and one coloumn is "gender".
Gender can be male or female but you will display "maennlich" or "weiblich" (german words for it - whatever...)

First step: Fetch into $Gender

Second:
define("male", "maennlich");
define("female", "weiblich");

Third:
echo constant($Gender);

Now, the index of the variable $Gender will be handled like a constant!

(It works like "echo male;" for better understanding)

And a result of this, it displays maennlich btw. weiblich!

greetz

[#13] Trevor Blackbird > yurab.com [2006-04-18 14:58:53]

Technically you can define constants with names that are not valid for variables:

<?php // $3some is not a valid variable name // This will not work $3some = 'invalid'; // This works define('3some', 'valid'); echo constant('3some'); ?>

Of course this is not a good practice, but PHP has got you covered.

[#14] [2005-10-11 07:20:21]

In reply to VGR_experts_exchange at edainworks dot com

To check if a constant is boolean, use this instead:

<?php if (TRACE === true) {} ?>

Much quicker and cleaner than using defined() and constant() to check for a simple boolean.

IMO, using ($var === true) or ($var === false) instead of ($var) or (!$var) is the best way to check for booleans no matter what. Leaves no chance of ambiguity.

[#15] Joachim Kruyswijk [2004-11-13 09:12:52]

The constant name can be an empty string.

Code:

define("", "foo");
echo constant("");

Output:

foo

[#16] VGR_experts_exchange at edainworks dot com [2003-09-19 05:32:21]

Hello. This applies to constants being defined as Boolean values, and may-be applies generally.

I advise you to NOT use this in an included file, in a function or elsewhere outside the scope where the define('TRACE',TRUE) is placed) :

if (TRACE) {}

This will always evaluate to TRUE if the constant is not defined previously (the story about this becoming an string 'TRACE', thus evaluating to TRUE)

Use this :

<?php if ((defined('TRACE'))AND(constant('TRACE'))) {} ?>

[#17] Andre [2003-04-27 13:10:37]

Maybe this is useful:

$file_ext is the file Extension of the image

<?php if ( imagetypes() & @constant('IMG_' . strtoupper($file_ext)) ) { $file_ext = $file_ext == 'jpg' ? 'jpeg' : $file_ext; $create_func = 'ImageCreateFrom' . $file_ext; } ?>