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6937b29491
We try to get rid of the myth that class constants were allocated for each instance, as claimed by user note 112286. Proof: <https://3v4l.org/3DKMC>. git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@339824 c90b9560-bf6c-de11-be94-00142212c4b1
155 lines
3.5 KiB
XML
155 lines
3.5 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<sect1 xml:id="language.oop5.constants" xmlns="http://docbook.org/ns/docbook">
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<title>Class Constants</title>
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<para>
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It is possible to define constant values on a per-class basis remaining the
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same and unchangeable. Constants differ from normal variables in that you
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don't use the <varname>$</varname> symbol to declare or use them.
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The default visibility of class constants is <literal>public</literal>.
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</para>
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<para>
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The value must be a constant expression, not (for example) a variable, a
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property, or a function call.
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</para>
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<para>
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It's also possible for interfaces to have <literal>constants</literal>. Look at
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the <link linkend="language.oop5.interfaces">interface documentation</link> for
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examples.
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</para>
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<para>
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As of PHP 5.3.0, it's possible to reference the class using a variable.
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The variable's value can not be a keyword (e.g. <literal>self</literal>,
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<literal>parent</literal> and <literal>static</literal>).
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</para>
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<para>
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Note that class constants are allocated once per class, and not for each
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class instance.
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</para>
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<example>
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<title>Defining and using a constant</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class MyClass
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{
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const CONSTANT = 'constant value';
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function showConstant() {
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echo self::CONSTANT . "\n";
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}
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}
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echo MyClass::CONSTANT . "\n";
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$classname = "MyClass";
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echo $classname::CONSTANT . "\n"; // As of PHP 5.3.0
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$class = new MyClass();
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$class->showConstant();
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echo $class::CONSTANT."\n"; // As of PHP 5.3.0
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?>
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]]>
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</programlisting>
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</example>
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<example>
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<title>Static data example</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class foo {
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// As of PHP 5.3.0
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const BAR = <<<'EOT'
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bar
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EOT;
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// As of PHP 5.3.0
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const BAZ = <<<EOT
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baz
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EOT;
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}
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?>
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]]>
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</programlisting>
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</example>
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<note>
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<para>
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Support for initializing constants with Heredoc and Nowdoc syntax was added in PHP 5.3.0.
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</para>
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</note>
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<example>
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<title>Constant expression example</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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const ONE = 1;
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class foo {
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// As of PHP 5.6.0
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const TWO = ONE * 2;
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const THREE = ONE + self::TWO;
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const SENTENCE = 'The value of THREE is '.self::THREE;
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}
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?>
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]]>
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</programlisting>
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<para>
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It is possible to provide a scalar expression involving numeric and string literals and/or constants in context of a class constant.
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</para>
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</example>
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<note>
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<para>
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Constant expression support was added in PHP 5.6.0.
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</para>
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</note>
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<example>
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<title>Class constant visibility modifiers</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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class Foo {
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// As of PHP 7.1.0
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public const BAR = 'bar';
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private const BAZ = 'baz';
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}
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echo Foo::BAR, PHP_EOL;
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echo Foo::BAR, PHP_EOL;
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?>
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]]>
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</programlisting>
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&example.outputs.71;
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<screen>
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<![CDATA[
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bar
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Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …
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]]>
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</screen>
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</example>
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<note>
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<para>
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As of PHP 7.1.0 visibility modifiers are allowed for class constants.
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</para>
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</note>
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</sect1>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-omittag:t
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sgml-shorttag:t
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sgml-minimize-attributes:nil
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sgml-always-quote-attributes:t
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sgml-indent-step:1
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sgml-indent-data:t
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indent-tabs-mode:nil
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sgml-parent-document:nil
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sgml-default-dtd-file:"~/.phpdoc/manual.ced"
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sgml-exposed-tags:nil
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sgml-local-catalogs:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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