PHP: Class Constants

Class Constants

It is possible to define constants on a per-class basis remaining the same and unchangeable. The default visibility of class constants is public.

Note:
Class constants can be redefined by a child class. As of PHP 8.1.0, class constants cannot be redefined by a child class if it is defined as final.

It's also possible for interfaces to have constants. Look at the interface documentation for examples.

It's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static).

Note that class constants are allocated once per class, and not for each class instance.

Example #1 Defining and using a constant

<?php
class MyClass
{
    const CONSTANT = 'constant value';

    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";
?>

The special ::class constant allows for fully qualified class name resolution at compile time, this is useful for namespaced classes:

Example #2 Namespaced ::class example

<?php
namespace foo {
    class bar {
    }

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

Example #3 Class constant expression example

<?php
const ONE = 1;
class foo {
    const TWO = ONE * 2;
    const THREE = ONE + self::TWO;
    const SENTENCE = 'The value of THREE is '.self::THREE;
}
?>

Example #4 Class constant visibility modifiers, as of 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;
?>

Output of the above example in PHP 7.1:

bar

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

Note:
As of PHP 7.1.0 visibility modifiers are allowed for class constants.

Example #5 Class constant visibility variance check, as of PHP 8.3.0

<?php

interface MyInterface
{
    public const VALUE = 42;
}

class MyClass implements MyInterface
{
    protected const VALUE = 42;
}
?>

Output of the above example in PHP 8.3:

Fatal error: Access level to MyClass::VALUE must be public (as in interface MyInterface) …

Note: As of PHP 8.3.0 visibility variance is checked more strictly. Prior to this version, the visibility of a class constant could be different from the visibility of the constant in the implemented interface.

Example #6 Fetch class constant syntax, as of PHP 8.3.0

<?php
class Foo {
    public const BAR = 'bar';
    private const BAZ = 'baz';
}

$name = 'BAR';
echo Foo::{$name}, PHP_EOL; // bar
?>

Note:
As of PHP 8.3.0, class constants can be fetched dynamically using a variable.

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User Contributed Notes 13 notes

down

206

tmp dot 4 dot longoria at gmail dot com ¶

13 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`"

?>

down

143

kuzawinski dot marcin at gmail dot com ¶

10 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 :)

down

151

anonymous ¶

14 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.

down

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
?>

down

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)
?>

down

jimmmy dot chief at gmail dot com ¶

8 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

?>

down

powtac at gmx dot de ¶

1 year 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)

down

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"

}

*/

?>

down

David Spector ¶

6 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.

down

Nimja ¶

7 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)

down

wbcarts at juno dot com ¶

16 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.

down

AamirSohailKmAs at gmail dot com ¶

9 hours ago

Note

as of PHP 8.3, it is possible to define Typed class constants.
RFC: https://wiki.php.net/rfc/typed_class_constants

<?php

class Example {
    public const string NAME = "Aamir";
    public const int  MAX    = 100;
}

?>

down

Paul ¶

9 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.

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