PHP 8.3.4 Released!

Klassenkonstanten

Es ist möglich, Konstanten für jede Klasse zu definieren, die gleich und unveränderlich bleiben. Die standardmäßige Sichtbarkeit einer Klassenkonstante ist public.

Hinweis:

Klassenkonstanten können von einer Kindklasse neu definiert werden. Ab PHP 8.1.0 können Klassenkonstanten nicht mehr von einer Kindklasse neu definiert werden, wenn sie als final definiert ist.

Ein Interface kann ebenfalls Konstanten enthalten. Die Interface-Dokumentation enthält Beispiele dazu.

Es ist möglich, eine Variable als Klassenreferenz zu nutzen. Der Variablenwert darf kein Schlüsselwort (wie self, parent oder static) sein.

Es ist zu beachten, dass Klassenkonstanten nur einmal pro Klasse angelegt werden und nicht für jedes Klassenexemplar.

Beispiel #1 Eine Konstante definieren und benutzen

<?php
class MyClass
{
const
CONSTANT = 'Konstanter Wert';

function
showConstant() {
echo
self::CONSTANT . "\n";
}
}

echo
MyClass::CONSTANT . "\n";

$classname = "MyClass";
echo
$classname::CONSTANT . "\n";

$class = new MyClass();
$class->showConstant();

echo
$class::CONSTANT."\n";
?>

Die spezielle Konstante ::class erlaubt die Auflösung voll qualifizierter Klassennamen zur Kompilierzeit, was nützlich für Klassen in Namensräumen ist:

Beispiel #2 Beispiel für ::class in einem Namensraum

<?php
namespace foo {
class
bar {
}

echo
bar::class; // foo\bar
}
?>

Beispiel #3 Beispiel für Ausdrücke mit Klassenkonstanten

<?php
const ONE = 1;
class
foo {
const
TWO = ONE * 2;
const
THREE = ONE + self::TWO;
const
SENTENCE = 'Der Wert von THREE ist '.self::THREE;
}
?>

Beispiel #4 Sichtbarkeitsmodifizierer von Klassenkonstanten seit PHP 7.1.0

<?php
class Foo {
public const
BAR = 'bar';
private const
BAZ = 'baz';
}
echo
Foo::BAR, PHP_EOL;
echo
Foo::BAZ, PHP_EOL;
?>

Das oben gezeigte Beispiel erzeugt mit PHP 7.1 folgende Ausgabe:

bar

Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …

Hinweis:

Von PHP 7.1.0 an sind Sichtbarkeitsmodifizierer für Klassenkonstanten erlaubt.

add a note

User Contributed Notes 12 notes

up
206
tmp dot 4 dot longoria at gmail dot com
12 years ago
it's possible to declare constant in base class, and override it in child, and access to correct value of the const from the static method is possible by 'get_called_class' method:
<?php
abstract class dbObject
{
const
TABLE_NAME='undefined';

public static function
GetAll()
{
$c = get_called_class();
return
"SELECT * FROM `".$c::TABLE_NAME."`";
}
}

class
dbPerson extends dbObject
{
const
TABLE_NAME='persons';
}

class
dbAdmin extends dbPerson
{
const
TABLE_NAME='admins';
}

echo
dbPerson::GetAll()."<br>";//output: "SELECT * FROM `persons`"
echo dbAdmin::GetAll()."<br>";//output: "SELECT * FROM `admins`"

?>
up
142
kuzawinski dot marcin at gmail dot com
9 years ago
As of PHP 5.6 you can finally define constant using math expressions, like this one:

<?php

class MyTimer {
const
SEC_PER_DAY = 60 * 60 * 24;
}

?>

Me happy :)
up
149
anonymous
13 years ago
Most people miss the point in declaring constants and confuse then things by trying to declare things like functions or arrays as constants. What happens next is to try things that are more complicated then necessary and sometimes lead to bad coding practices. Let me explain...

A constant is a name for a value (but it's NOT a variable), that usually will be replaced in the code while it gets COMPILED and NOT at runtime.

So returned values from functions can't be used, because they will return a value only at runtime.

Arrays can't be used, because they are data structures that exist at runtime.

One main purpose of declaring a constant is usually using a value in your code, that you can replace easily in one place without looking for all the occurences. Another is, to avoid mistakes.

Think about some examples written by some before me:

1. const MY_ARR = "return array(\"A\", \"B\", \"C\", \"D\");";
It was said, this would declare an array that can be used with eval. WRONG! This is just a string as constant, NOT an array. Does it make sense if it would be possible to declare an array as constant? Probably not. Instead declare the values of the array as constants and make an array variable.

2. const magic_quotes = (bool)get_magic_quotes_gpc();
This can't work, of course. And it doesn't make sense either. The function already returns the value, there is no purpose in declaring a constant for the same thing.

3. Someone spoke about "dynamic" assignments to constants. What? There are no dynamic assignments to constants, runtime assignments work _only_ with variables. Let's take the proposed example:

<?php
/**
* Constants that deal only with the database
*/
class DbConstant extends aClassConstant {
protected
$host = 'localhost';
protected
$user = 'user';
protected
$password = 'pass';
protected
$database = 'db';
protected
$time;
function
__construct() {
$this->time = time() + 1; // dynamic assignment
}
}
?>

Those aren't constants, those are properties of the class. Something like "this->time = time()" would even totally defy the purpose of a constant. Constants are supposed to be just that, constant values, on every execution. They are not supposed to change every time a script runs or a class is instantiated.

Conclusion: Don't try to reinvent constants as variables. If constants don't work, just use variables. Then you don't need to reinvent methods to achieve things for what is already there.
up
115
delete dot this dot and dot dots dot gt at kani dot hu
10 years ago
I think it's useful if we draw some attention to late static binding here:
<?php
class A {
const
MY_CONST = false;
public function
my_const_self() {
return
self::MY_CONST;
}
public function
my_const_static() {
return static::
MY_CONST;
}
}

class
B extends A {
const
MY_CONST = true;
}

$b = new B();
echo
$b->my_const_self ? 'yes' : 'no'; // output: no
echo $b->my_const_static ? 'yes' : 'no'; // output: yes
?>
up
93
Xiong Chiamiov
10 years ago
const can also be used directly in namespaces, a feature never explicitly stated in the documentation.

<?php
# foo.php
namespace Foo;

const
BAR = 1;
?>

<?php
# bar.php
require 'foo.php';

var_dump(Foo\BAR); // => int(1)
?>
up
47
jimmmy dot chief at gmail dot com
7 years ago
Hi, i would like to point out difference between self::CONST and $this::CONST with extended class.
Let us have class a:

<?php
class a {
const
CONST_INT = 10;

public function
getSelf(){
return
self::CONST_INT;
}

public function
getThis(){
return
$this::CONST_INT;
}
}
?>

And class b (which extends a)

<?php
class b extends a {
const
CONST_INT = 20;

public function
getSelf(){
return
parent::getSelf();
}

public function
getThis(){
return
parent::getThis();
}
}
?>

Both classes have same named constant CONST_INT.
When child call method in parent class, there is different output between self and $this usage.

<?php
$b
= new b();

print_r($b->getSelf()); //10
print_r($b->getThis()); //20

?>
up
29
nepomuk at nepda dot de
8 years ago
[Editor's note: that is already possible as of PHP 5.6.0.]

Note, as of PHP7 it is possible to define class constants with an array.

<?php
class MyClass
{
const
ABC = array('A', 'B', 'C');
const
A = '1';
const
B = '2';
const
C = '3';
const
NUMBERS = array(
self::A,
self::B,
self::C,
);
}
var_dump(MyClass::ABC);
var_dump(MyClass::NUMBERS);

// Result:
/*
array(3) {
[0]=>
string(1) "A"
[1]=>
string(1) "B"
[2]=>
string(1) "C"
}
array(3) {
[0]=>
string(1) "1"
[1]=>
string(1) "2"
[2]=>
string(1) "3"
}
*/
?>
up
1
powtac at gmx dot de
10 months ago
Since it it is not mentioned in the description the following types can be set as a class constant: string, array, int, bool and probably float. But not objects.

<?php

class Test {
const
arr = array();
const
string = 'string';
const
int = 99;
const
bool = true;
}

var_dump(
(new
Test())::arr,
(new
Test())::string,
(new
Test())::int,
(new
Test())::bool
);

/* ouput for PHP 7.0.0+:

array(0) {
}
string(6) "string"
int(99)
bool(true)
up
3
Nimja
6 years ago
Note that this magic constant DOES NOT load classes. And in fact can work on classes that do not exist.

This means it does not mess with auto-loading.

<?php
$className
= \Foo\Bar::class;
var_dump($className);
var_dump(class_exists($className, false));
?>

Will output:

string(7) "Foo\Bar"
bool(false)
up
9
wbcarts at juno dot com
15 years ago
Use CONST to set UPPER and LOWER LIMITS

If you have code that accepts user input or you just need to make sure input is acceptable, you can use constants to set upper and lower limits. Note: a static function that enforces your limits is highly recommended... sniff the clamp() function below for a taste.

<?php

class Dimension
{
const
MIN = 0, MAX = 800;

public
$width, $height;

public function
__construct($w = 0, $h = 0){
$this->width = self::clamp($w);
$this->height = self::clamp($h);
}

public function
__toString(){
return
"Dimension [width=$this->width, height=$this->height]";
}

protected static function
clamp($value){
if(
$value < self::MIN) $value = self::MIN;
if(
$value > self::MAX) $value = self::MAX;
return
$value;
}
}

echo (new
Dimension()) . '<br>';
echo (new
Dimension(1500, 97)) . '<br>';
echo (new
Dimension(14, -20)) . '<br>';
echo (new
Dimension(240, 80)) . '<br>';

?>

- - - - - - - -
Dimension [width=0, height=0] - default size
Dimension [width=800, height=97] - width has been clamped to MAX
Dimension [width=14, height=0] - height has been clamped to MIN
Dimension [width=240, height=80] - width and height unchanged
- - - - - - - -

Setting upper and lower limits on your classes also help your objects make sense. For example, it is not possible for the width or height of a Dimension to be negative. It is up to you to keep phoney input from corrupting your objects, and to avoid potential errors and exceptions in other parts of your code.
up
3
Paul
8 years ago
Square or curly bracket syntax can normally be used to access a single byte (character) within a string. For example: $mystring[5]. However, please note that (for some reason) this syntax is not accepted for string class constants (at least, not in PHP 5.5.12).
For example, the following code gives "PHP Parse error: syntax error, unexpected '[' in php shell code on line 6".
<?php
class SomeClass
{
const
SOME_STRING = '0123456790';
public static function
ATest()
{
return
self::SOME_STRING[0];
}
}
?>
It looks like you have to use a variable/class member instead.
up
0
David Spector
5 years ago
The usual comma-separated syntax can be used to declare several constants:

class STATE
{
const INIT=0, NAME_SEEN=1, ADDR_SEEN=2;
}

This shows the declaration of a set of enumeration literals suitable for use in a finite state machine loop. Reference such an enum by using syntax such as "STATE::INIT". Its actual type in this case will be integer.
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