## Class Constants

It is possible to define constant values on a per-class basis remaining the same and unchangeable. Constants differ from normal variables in that you don't use the $symbol to declare or use them. The default visibility of class constants is public. The value must be a constant expression, not (for example) a variable, a property, or a function call. It's also possible for interfaces to have constants. Look at the interface documentation for examples. As of PHP 5.3.0, it's possible to reference the class using a variable. The variable's value can not be a keyword (e.g. self, parent and static). Note that class constants are allocated once per class, and not for each class instance. Example #1 Defining and using a constant <?php class MyClass { const CONSTANT 'constant value'; function showConstant() { echo self::CONSTANT "\n"; } } echo MyClass::CONSTANT "\n";$classname "MyClass";
echo
$classname::CONSTANT "\n"// As of PHP 5.3.0$class = new MyClass();
$class->showConstant(); echo$class::CONSTANT."\n"// As of PHP 5.3.0
?>

Example #2 Static data example

<?php
class foo {

// As of PHP 5.3.0

const BAR = <<<'EOT'
bar
EOT;

// As of PHP 5.3.0

const BAZ = <<<EOT
baz
EOT;
}
?>

Note:

Support for initializing constants with Heredoc and Nowdoc syntax was added in PHP 5.3.0.

The special ::class constant are available as of PHP 5.5.0, and allows for fully qualified class name resolution at compile, this is useful for namespaced classes:

Example #3 Namespaced ::class example

<?php
namespace foo {
class
bar {
}

echo
bar::class; // foo\bar
}
?>

Example #4 Constant expression example

<?php
const ONE 1;

class
foo {

// As of PHP 5.6.0

const TWO ONE 2;
const
THREE ONE self::TWO;
const
SENTENCE 'The value of THREE is '.self::THREE;
}
?>

It is possible to provide a scalar expression involving numeric and string literals and/or constants in context of a class constant.

Note:

Constant expression support was added in PHP 5.6.0.

Example #5 Class constant visibility modifiers

<?php
class Foo {

// As of PHP 7.1.0

public const BAR 'bar';
private const
BAZ 'baz';
}
echo
Foo::BARPHP_EOL;
echo
Foo::BAZPHP_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.

### User Contributed Notes 19 notes

136
tmp dot 4 dot longoria at gmail dot com
7 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
{
const
}

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

?>
107
kuzawinski dot marcin at gmail dot com
3 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 :)
98
anonymous
7 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.
74
delete dot this dot and dot dots dot gt at kani dot hu
4 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 $b->my_const_static ? 'yes' : 'no'; // output: yes ?> 69 Xiong Chiamiov 4 years ago const can also be used directly in namespaces, a feature never explicitly stated in the documentation. <?php # foo.php namespace Foo; const BAR = 1; ?> <?php # bar.php require 'foo.php'; var_dump(Foo\BAR); // => int(1) ?> 14 jimmmy dot chief at gmail dot com 1 year ago Hi, i would like to point out difference between self::CONST and$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($b->getThis()); //20 ?> Nimja 10 months ago Note that this magic constant DOES NOT load classes. And in fact can work on classes that do not exist. This means it does not mess with auto-loading. <?php$className
= \Foo\Bar::class;

var_dump($className); var_dump(class_exists($className, false));
?>

Will output:

string(7) "Foo\Bar"
bool(false)
11
nepomuk at nepda dot de
2 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"
}
*/
?>
Paul
3 years ago
Square or curly bracket syntax can normally be used to access a single byte (character) within a string. For example: $mystring[5]. However, please note that (for some reason) this syntax is not accepted for string class constants (at least, not in PHP 5.5.12). For example, the following code gives "PHP Parse error: syntax error, unexpected '[' in php shell code on line 6". <?php class SomeClass { const SOME_STRING = '0123456790'; public static function ATest() { return self::SOME_STRING[0]; } } ?> It looks like you have to use a variable/class member instead. keenskelly at gmail dot com 4 years ago Re: "The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call." I dare say that "a mathematical operation" can indeed be a constant expression. I was quite surprised by this limitation; you cannot, for example do something like: const LIMITMB = 20; const LIMITB = LIMITMB * 1024 * 1024; It is very common to be able to express something like that in other languages, like C with #defines, where changing one definition has a cascading effect on others without having to pre-calculate hard-coded numbers all over the place. So beware, you might be better off using a private static or global contstant definition if you need to do anything more sophisticated than a name=primitive value pair. 10 wbcarts at juno dot com 9 years ago Use CONST to set UPPER and LOWER LIMITS If you have code that accepts user input or you just need to make sure input is acceptable, you can use constants to set upper and lower limits. Note: a static function that enforces your limits is highly recommended... sniff the clamp() function below for a taste. <?php class Dimension { const MIN = 0, MAX = 800; public$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
$value; } } echo (new Dimension()) . '<br>'; echo (new Dimension(1500, 97)) . '<br>'; echo (new Dimension(14, -20)) . '<br>'; echo (new Dimension(240, 80)) . '<br>'; ?> - - - - - - - - Dimension [width=0, height=0] - default size Dimension [width=800, height=97] - width has been clamped to MAX Dimension [width=14, height=0] - height has been clamped to MIN Dimension [width=240, height=80] - width and height unchanged - - - - - - - - Setting upper and lower limits on your classes also help your objects make sense. For example, it is not possible for the width or height of a Dimension to be negative. It is up to you to keep phoney input from corrupting your objects, and to avoid potential errors and exceptions in other parts of your code. info at stanzentech dot com 3 years ago <?php //http://php.net/manual/en/language.oop5.constants.php /** * Constant name shouldn't start with$
* Constant name may lower or uppercases.
* Same constant name can be used as a property name but must start with $symbol. * Constant doesn't available with$this-> inside class definition.
* Constant is available with self:: inside class definition.
* Constant can't call with $this-> outside class. * Constant is accessible with :: after "Class Name or Object". * */ class MyClass { // Parse error: syntax error, unexpected '$CONSTANT' (T_VARIABLE), expecting identifier (T_STRING) in constant.php
//const $CONSTANT = 'constant named "CONSTANT" '; const CONSTANT = 'constant named "CONSTANT" '; const small = 'constant named "small" '; public$small        =   'SAME CONTSNAT NAME AS PROPERTIES.'

//Fatal error: Cannot redefine class constant MyClass::small in constant.php
// const small         =   'constant named "small" ';

function showConstant() {
echo
self::CONSTANT . "<br>";

//echo $this->CONSTANT . "<br>"; // Notice: Undefined property: MyClass::$CONSTANT in constant.php

}
}

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

//Notice: Undefined property: MyClass::$CONSTANT in constant.php //echo$class->CONSTANT."<br>";

echo $class->small."<br>"// SAME CONTSNAT NAME AS PROPERTIES. ?> dubious 4 months ago If absolutely necessary you can set class constants to variables like this:$temp_wfr = substr(dirname(__FILE__),0,-3); // temp site root directory
$toeval = <<<TOEVAL class Server extends Server_Base { const LOCATION = "development"; const WEBFILEROOT = "$temp_wfr";
}
TOEVAL;
eval($toeval); While I wouldn't use this in production, it came in very handy in my development environment where my directory locations were changing with each test version of the site being developed. Server::WEBFILEROOT was used throughout the site, so setting it automatically saved me a lot of time. scohen987 at gmail dot com 10 months ago All non-class constants, declared or not, also have a magic class constant, ie: >>> FOO PHP error: Use of undefined constant FOO - assumed 'FOO' on line 1 >>> FOO::class => "FOO" >>> define('FOO','BAR') => true >>> FOO => "BAR" >>> FOO::class => "FOO" Anonymous 11 months ago Before version 7.1.0 you couldn't write something like this: <?php class foo{ private const myvar = 13; ... } ?> and an if with version_compare isn't also allowed how could i write the code for it so the class is compatible from 5.6 - 7.1.0? Also I don't understand which are the advantages of modifying the class scope of a constant because constant couldn't be change at runtime. Maybe someone could clarify this Anonymous 4 years ago Noted by another is that class constants take up memory for every instance. I cannot see this functionality being accurate, so testing thusly: class SomeClass { const thing = 0; const thing2 = 1; }$m0 = memory_get_usage();
$p0 = new SomeClass();$p1 = new SomeClass();
$p2 = new SomeClass();$m1 = memory_get_usage();
printf("memory %d<br />", $m1 -$m0);

The output does not change when one alters the count of constants in "SomeClass".
-1
enrico_kaelert at kabelmail dot com
4 years ago
additional to tmp dot 4 dot longoria at gmail dot com ´s post:
quote:
it's possible to declare constant in base class, and override it in child,
/quote

Its not that we overwrite them.
Its more that each got its own:
<?php
abstract class dbObject
{
const
TABLE_NAME='undefined';
}

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

public static function
getSelf()
{
return
self::TABLE_NAME;
}
public static function
getParent()
{
return
parent::TABLE_NAME;
}
}

class
{
const

public static function
getSelf()
{
return
self::TABLE_NAME;
}
public static function
getParent()
{
return
parent::TABLE_NAME;
}
}

echo
'<pre>
im class dbPerson{} and this is my:
self TABLE_NAME:    '
.dbPerson::getSelf().'   // persons
parent TABLE_NAME: '
.dbPerson::getParent().'  // undefined

im class dbAdmin{} and this is my:
self TABLE_NAME:   '
parent TABLE_NAME: '
'
;
?>

<?php
class ParentClass
{
const
CONSTANT = 'CONST_PARENT';
}

class
A extends ParentClass
{
const
CONSTANT = 'CONST_A';

public static function
getSelf()
{
return
self::CONSTANT;
}
public static function
getParent()
{
return
parent::CONSTANT;
}
}

echo
'<pre>
im class A{} and this is my:
self CONSTANT:    '
.A::getSelf().'   // CONST_A
parent CONSTANT: '
.A::getParent().'  // CONST_PARENT
'
;
?>
-7
jakub dot lopuszanski at nasza-klasa dot pl
7 years ago
[Editor's note: that behavior has changed as of PHP 7.0.0, though.]

Suprisingly consts are lazy bound even though you use self instead of static:
<?php
class A{
const
X=1;
const
Y=self::X;
}
class
B extends A{
const
X=1.0;
}
var_dump(B::Y); // float(1.0)
?>
-8
jaimz at vertigolabs dot org
4 years ago
I thought it would be relevant to point out that with php 5.5, you can not use self::class, static::class, or parent::class to produce a FQN. Doing so produces a PHP Parse error:

"PHP Parse error:  syntax error, unexpected 'class' (T_CLASS), expecting variable (T_VARIABLE) or '\$'"

It would be nice if you could do this however.