language/oop5/constants.xml
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fc174e8d6162091550edde46159917ee7e5a2e73
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<?xml version="1.0" encoding="utf-8"?>
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<?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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</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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It is possible to define <link linkend="language.constants">constants</link>
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on a per-class basis remaining the same and unchangeable.
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The default visibility of class constants is <literal>public</literal>.
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</para>
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<note>
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<para>
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Class constants can be redefined by a child class.
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As of PHP 8.1.0, class constants cannot be redefined by a child class
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if it is defined as <link linkend="language.oop5.final">final</link>.
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</para>
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</note>
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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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It's also possible for interfaces to have constants. Look
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at the <link linkend="language.oop5.interfaces">interface documentation</link>
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for 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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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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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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echo $classname::CONSTANT . "\n";
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$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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echo $class::CONSTANT."\n";
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?>
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]]>
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</programlisting>
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</example>
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<para>
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The special <constant>::class</constant> constant allows
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for fully qualified class name resolution at compile time,
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this is useful for namespaced classes:
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</para>
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<example>
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<title>Static data example</title>
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<title>Namespaced ::class 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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namespace foo {
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class bar {
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}
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echo bar::class; // foo\bar
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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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<title>Class 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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</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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<example>
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<title>Class constant visibility modifiers, as of PHP 7.1.0</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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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::BAZ, 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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Constant expression support was added in PHP 5.6.0.
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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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