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constant

(PHP 4 >= 4.0.4, PHP 5)

constantLiefert den Wert einer Konstante

Beschreibung

mixed constant ( string $name )

Gibt den Wert der durch name angegebenen Konstante zurück.

constant() ist nützlich, um den Wert einer Konstante abzufragen, deren Namen Sie nicht kennen, d.h. wenn sie in einer Variablen gespeichert ist oder von einer Funktion zurückgegeben wird.

Diese Funktion funktioniert auch mit Klassenkonstanten.

Parameter-Liste

name

Der Name der Konstante

Rückgabewerte

Gibt den Wert der Konstante zurück, oder NULL, falls die Konstante nicht definiert ist.

Beispiele

Beispiel #1 constant()-Beispiel

<?php

define
("MAXGROESSE"100);

echo 
MAXGROESSE;
echo 
constant("MAXGROESSE"); // dasselbe wie in der vorigen Zeile



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!"

?>

Siehe auch


18 BenutzerBeiträge:
- Beiträge aktualisieren...
bohwaz
5.07.2010 15:28
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
25.05.2010 17:07
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
10.04.2010 8:41
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
21.06.2009 17:21
howto echo CONSTANT_NAME without warnings and "if " checking:

<?php

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

?>
cory dot mawhorter @ ephective dot com
8.09.2008 23:08
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
15.02.2008 12:34
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
1.03.2007 14:57
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

2.02.2007 5:29
@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
19.01.2007 11:13
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.

4.10.2006 2:17
If the constant does not exist, constant() will generate a warning and return null.
narada dot sage at googlemail dot com
13.07.2006 15:01
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
16.05.2006 19:00
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
18.04.2006 23:58
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
26.11.2005 9:39
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);
    }
}

11.10.2005 16:20
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
13.11.2004 18:12
The constant name can be an empty string.

Code:

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

Output:

foo
VGR_experts_exchange at edainworks dot com
19.09.2003 14:32
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
27.04.2003 22:10
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;
}
?>



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