php-doc-en/language/oop5/constants.xml

<?xml version="1.0" encoding="utf-8"?>
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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.
  </para>
 </note>
 <para>
  It's also possible for interfaces to have <literal>constants</literal>. 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>
</sect1>
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