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<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<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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