archived-doc-zh/reference/misc/functions/constant.xml

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<refentry xml:id="function.constant" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>constant</refname>
  <refpurpose>返回一个常量的值</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>mixed</type><methodname>constant</methodname>
   <methodparam><type>string</type><parameter>name</parameter></methodparam>
  </methodsynopsis>
  <simpara>
   返回 <parameter>name</parameter> 对应的常量的值。
  </simpara>
  <simpara>
   当你不知道常量名，却需要获取常量的值时，<function>constant</function> 就很有用了。也就是说，常量名储存在一个变量里，或者由函数返回时。
  </simpara>
  <simpara>
   该函数也适用<link
   linkend="language.oop5.constants">类常量</link>和 <link
   linkend="language.types.enumerations">enum cases</link>。
  </simpara>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>name</parameter></term>
     <listitem>
      <para>
          常量名。
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
  返回常量的值。
  </para>
 </refsect1>

 <refsect1 role="errors">
  &reftitle.errors;
  <para>
   如果常量未定义，会抛出 <classname>Error</classname> 异常。
   在 PHP 8.0.0 之前，会产生 <constant>E_WARNING</constant> 级别的错误。
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <informaltable>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>&Version;</entry>
      <entry>&Description;</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>8.0.0</entry>
      <entry>
       如果常量未定义，<function>constant</function> 现在会抛出
       <classname>Error</classname> 异常。以前会产生一个
       <constant>E_WARNING</constant> 级别的错误并返回 &null;。
      </entry>
     </row>
    </tbody>
   </tgroup>
  </informaltable>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>constant</function> 与常量一起使用</title>
    <programlisting role="php">
<![CDATA[
<?php

define("MAXSIZE", 100);

echo MAXSIZE;
echo constant("MAXSIZE"); // 和上行一样


interface bar {
    const test = 'foobar!';
}

class foo {
    const test = 'foobar!';
}

$const = 'test';

var_dump(constant('bar::'. $const)); // string(7) "foobar!"
var_dump(constant('foo::'. $const)); // string(7) "foobar!"

?>
]]>
    </programlisting>
   </example>
   <example>
    <title><function>constant</function> 和 Enum Cases 一起使用（自 PHP 8.1.0 起）</title>
    <programlisting role="php">
<![CDATA[
<?php

enum Suit
{
    case Hearts;
    case Diamonds;
    case Clubs;
    case Spades;
}

$case = 'Hearts';

var_dump(constant('Suit::'. $case)); // enum(Suit::Hearts)

?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>define</function></member>
    <member><function>defined</function></member>
    <member><function>get_defined_constants</function></member>
    <member>关于 <link linkend="language.constants">常量</link> 的章节</member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

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