5.5.15

constant

(PHP 4 >= 4.0.4, PHP 5)

constantReturns the value of a constant

Descrizione

mixed constant ( string $name )

Return the value of the constant indicated by name.

constant() is useful if you need to retrieve the value of a constant, but do not know its name. I.e. it is stored in a variable or returned by a function.

This function works also with class constants.

Elenco dei parametri

name

The constant name.

Valori restituiti

Returns the value of the constant, or NULL if the constant is not defined.

Errori/Eccezioni

An E_WARNING level error is generated if the constant is not defined.

Esempi

Example #1 constant() example

<?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!"

?>

Vedere anche:

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User Contributed Notes 16 notes

up
7
Andre
11 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;
}
?>
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8
Joachim Kruyswijk
9 years ago
The constant name can be an empty string.

Code:

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

Output:

foo
up
2
Trevor Blackbird > yurab.com
8 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
2
XC
7 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
1
narada dot sage at googlemail dot com
8 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
0
Richard J. Turner
1 year 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
0
bohwaz
4 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
0
Anonymous
8 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
VGR_experts_exchange at edainworks dot com
10 years ago
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')))  {}
?>
up
-1
adam at adamhahn dot com
2 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
-2
cory dot mawhorter @ ephective dot com
5 years ago
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.
up
-1
hellekin
4 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
-2
Anonymous
7 years ago
@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') {
// ...
}

?>
up
-1
dachnik
4 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
service at dual-creators dot de
8 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
-3
roller
5 years ago
howto echo CONSTANT_NAME without warnings and "if " checking:

<?php

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

?>
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