constant

(PHP 4 >= 4.0.4, PHP 5, PHP 7)

constantВозвращает значение константы

Описание

mixed constant ( string $name )

Возвращает значение константы, указанной в параметре name.

Функция constant() полезна, если вам необходимо получить значение константы, но неизвестно её имя. Например, если оно хранится в переменной или возвращается функцией.

Данная функция также работает с константами классов.

Список параметров

name

Имя константы.

Возвращаемые значения

Возвращает значение константы либо NULL, если константа не определена.

Ошибки

Если константа не определена, то генерируется исключение уровня E_WARNING.

Примеры

Пример #1 Пример функции constant()

<?php

define
("MAXSIZE"100);

echo 
MAXSIZE;
echo 
constant("MAXSIZE"); // результат аналогичен предыдущему выводу


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!"

?>

Смотрите также

  • define() - Определяет именованную константу
  • defined() - Проверяет существование указанной именованной константы
  • См. раздел Константы

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User Contributed Notes 16 notes

up
45
Joachim Kruyswijk
13 years ago
The constant name can be an empty string.

Code:

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

Output:

foo
up
14
t dot kmieliauskas at gmail dot com
2 years ago
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)
up
4
helvete at zhouba dot cz
1 year ago
It is worth noting, that keyword 'self' can be used for constant retrieval from within the class it is defined

<?php
class Foo {
  const
PARAM_BAR = 'baz';

  public function
getConst($name) {
    return
constant("self::{$name}");
  }
}

$foo = new Foo();
echo
$foo->getConst('PARAM_BAR'); // prints 'baz'
?>
up
14
Richard J. Turner
4 years ago
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.
up
9
Andre
14 years ago
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;
}
?>
up
10
Trevor Blackbird > yurab.com
11 years ago
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.
up
1
mohammad alzoqaily
2 months ago
// 1)  you can store the name of constant in default variable
//     and use it without identify it's name :)

        $str= "constName";
     
        define("constName","this is constant");

        echo constant($str);
      

       output:
             this is constant

// 2)  good for dynamic generating  constants
      

         function generateConst( $const ,  $value , $sensitivity=TRUE )
             {
             
                    define( "$const" , "$value ",$sensitivity);
              }

              $CONST="cost";
              $VALUE="100$";
             
             generateConst( $CONST , $VALUE);
                         
             echo constant($const);

        output:
                100$
up
4
XC
10 years ago
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.
up
4
narada dot sage at googlemail dot com
11 years ago
To access the value of a class constant use the following technique.

<?php

class a {
    const
b = 'c';
}

echo
constant('a::b');

// output: c

?>
up
2
Anonymous
12 years ago
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.
up
-1
adam at adamhahn dot com
6 years ago
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.
up
-1
hellekin
7 years ago
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
?>
up
-3
bohwaz
7 years ago
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!';
}

?>
up
-3
service at dual-creators dot de
11 years ago
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
up
-2
dachnik
7 years ago
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
up
-3
ss
2 months ago
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