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<sect1 xml:id="language.oop5.constants" xmlns="http://docbook.org/ns/docbook">

<title>Class Constants</title>

<para>

It is possible to define <link linkend="language.constants">constants</link>

on a per-class basis remaining the same and unchangeable.

The default visibility of class constants is <literal>public</literal>.

</para>

<note>

<para>

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 <link linkend="language.oop5.final">final</link>.

</para>

</note>

<para>

It's also possible for interfaces to have constants. Look

at the <link linkend="language.oop5.interfaces">interface documentation</link>

for examples.

</para>

<para>

It's possible to reference the class using a variable.

The variable's value can not be a keyword (e.g. <literal>self</literal>,

<literal>parent</literal> and <literal>static</literal>).

</para>

<para>

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

class instance.

</para>

<example>

<title>Defining and using a constant</title>

<programlisting role="php">

<![CDATA[

<?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";

?>

]]>

</programlisting>

</example>

<para>

The special <constant>::class</constant> constant allows

for fully qualified class name resolution at compile time,

this is useful for namespaced classes:

</para>

<example>

<title>Namespaced ::class example</title>

<programlisting role="php">

<![CDATA[

<?php

namespace foo {

class bar {

}

echo bar::class; // foo\bar

}

?>

]]>

</programlisting>

</example>

<example>

<title>Class constant expression example</title>

<programlisting role="php">

<![CDATA[

<?php

const ONE = 1;

class foo {

const TWO = ONE * 2;

const THREE = ONE + self::TWO;

const SENTENCE = 'The value of THREE is '.self::THREE;

}

?>

]]>

</programlisting>

</example>

<example>

<title>Class constant visibility modifiers, as of PHP 7.1.0</title>

<programlisting role="php">

<![CDATA[

<?php

class Foo {

public const BAR = 'bar';

private const BAZ = 'baz';

}

echo Foo::BAR, PHP_EOL;

echo Foo::BAZ, PHP_EOL;

?>

]]>

</programlisting>

&example.outputs.71;

<screen>

<![CDATA[

bar

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

]]>

</screen>

</example>

<note>

<para>

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

</para>

</note>

<example>

<title>Class constant visibility variance check, as of PHP 8.3.0</title>

<programlisting role="php">

<![CDATA[

<?php

interface MyInterface

{

public const VALUE = 42;

}

class MyClass implements MyInterface

{

protected const VALUE = 42;

}

?>

]]>

</programlisting>

&example.outputs.83;

<screen>

<![CDATA[

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

]]>

</screen>

</example>

<note>

<simpara>

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.

</simpara>

</note>

<example>

<title>Fetch class constant syntax, as of PHP 8.3.0</title>

<programlisting role="php">

<![CDATA[

<?php

class Foo {

public const BAR = 'bar';

private const BAZ = 'baz';

}

$name = 'BAR';

echo Foo::{$name}, PHP_EOL; // bar

?>

]]>

</programlisting>

</example>

<note>

<para>

As of PHP 8.3.0, class constants can be fetched dynamically using a

variable.

</para>

</note>

</sect1>

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