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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

杂项 函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 返回一个常量的值

用户评论:

adam at adamhahn dot com (28-Sep-2011 08:59)

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.

Anonymous (08-Mar-2011 02:59)

You can get the value of class and interface constants using this function.

<?php
interface IDatabase {
   const Test = 22;
}

class MyDatabase implements IDatabase {

}

echo constant('MyDatabase::Test'); # Outputs 22
echo constant('IDatabase::Test'); # Also outputs 22
?>

bohwaz (05-Jul-2010 02:28)

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!';
}

?>

hellekin (25-May-2010 04:07)

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
?>

dachnik (10-Apr-2010 07:41)

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

roller (21-Jun-2009 04:21)

howto echo CONSTANT_NAME without warnings and "if " checking:

<?php

!defined("CONSTANT_NAME") || constant("CONSTANT_NAME");

?>

cory dot mawhorter @ ephective dot com (08-Sep-2008 10:08)

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.

Joachim Chmielewski (15-Feb-2008 11:34)

To use constants in functions additional its not neccessary to change the function call wherever you need the constant in a function.

   foo($var1);

to

    foo($var1,$const);

More easy is to change the function definition and use a default value for a new variable that contains the const value.

   function foo($var1,$my_const=CONST_VALUE){
          if($my_const==1) dosomething();
   }

Now it`s not necessary to change the function call wherever you need the function.

vgr at europeanexperts dot org (01-Mar-2007 01:57)

in reply to anonymous

[quote]
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.
[/quote]

is definitely nor cleaner (because it's still as wrong as using simply "if (TRACE)") nor quicker than " if (TRACE)" (one more comparison on a boolean value). This will generate PHP errors. The constant TRACE is NOT defined.

error :
PHP Notice:  Use of undefined constant TRACE - assumed 'TRACE' in yourpath/test_constants.php on line 5

if you really want to be "clean" and as quick as possible, then there is a function :

[code]
function IsBooleanConstantAndTrue($constname) { // : Boolean
  $res=FALSE;
  if (defined($constname)) $res=(constant($constname)===TRUE);
  return($res);
}

// use : if (IsBooleanConstantAndTrue('TRACE')) echo "trace is really True<br>";
[/code]

If you want, you can see a demonstration at http://www.fecj.org/extra/test_constants.php

Regards

(02-Feb-2007 04:29)

@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') {
// ...
}

?>

XC (19-Jan-2007 10:13)

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.

(04-Oct-2006 01:17)

If the constant does not exist, constant() will generate a warning and return null.

narada dot sage at googlemail dot com (13-Jul-2006 02:01)

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

<?php

class a {
    const b = 'c';
}

echo constant('a::b');

// output: c

?>

service at dual-creators dot de (16-May-2006 06:00)

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

Trevor Blackbird > yurab.com (18-Apr-2006 10:58)

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.

timneill at hotmail dot com (26-Nov-2005 08:39)

Please note when using this function from within a class to retrieve a php5 class constant, ensure you include the 'self::'.

class Validate
{
    const TEXT_MAX = 65536;
   
    //-- this will work
    public static function textWORKS($_value, $_type = 'TEXT')
    {
        $_max = constant('self::' . $_type . '_MAX');
        return (strlen($_value) <= $_max ? true : false);
    }
   
    //-- this will fail
    public static function textFAILS($_value, $_type = 'TEXT')
    {
        //-- Debug Warning: constant(): Couldn't find constant TEXT_MAX
        $_max = constant($_type . '_MAX');
        return (strlen($_value) <= $_max ? true : false);
    }
}

(11-Oct-2005 03:20)

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.

Joachim Kruyswijk (13-Nov-2004 05:12)

The constant name can be an empty string.

Code:

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

Output:

foo

VGR_experts_exchange at edainworks dot com (19-Sep-2003 01:32)

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')))  {}
?>

Andre (27-Apr-2003 09:10)

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;
}
?>