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276 lines
6.7 KiB
XML
Executable File
276 lines
6.7 KiB
XML
Executable File
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<!-- EN-Revision: eaec4ab10a65c4515ee2fb899d06e89bae3754b0 Maintainer: daijie Status: ready -->
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<!-- CREDITS: mowangjuanzi -->
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<refentry xml:id="function.list" xmlns="http://docbook.org/ns/docbook">
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<refnamediv>
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<refname>list</refname>
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<refpurpose>把数组中的值赋给一组变量 </refpurpose>
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</refnamediv>
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<refsect1 role="description">
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&reftitle.description;
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<methodsynopsis>
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<type>array</type><methodname>list</methodname>
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<methodparam><type>mixed</type><parameter>var</parameter></methodparam>
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<methodparam rep="repeat" choice="opt"><type>mixed</type><parameter>vars</parameter></methodparam>
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</methodsynopsis>
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<para>
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像 <function>array</function> 一样，这不是真正的函数，而是语言结构。
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<function>list</function> 可以在单次操作内为一组变量赋值。仅数组和实现了
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<link linkend="class.arrayaccess">ArrayAccess</link>
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的对象可以解包。<function>list</function> 表达式不能为空。
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</para>
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<note>
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<para>
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在 PHP 7.1.0 之前的版本，<function>list</function> 仅能用于数字索引的数组，并假定数字索引从 0 开始。
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</para>
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</note>
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<para>
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自 PHP 7.1.0 起，<function>list</function> 还可以包含指定的 key，从而允许使用非整数或非连续 key
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解构数组。有关数组解构的更多详细信息，请参阅<link linkend="language.types.array.syntax.destructuring">数组解构部分</link>。
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</para>
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<note>
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<para>
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尝试访问未定义的数组 key 与访问其他未定义变量相同：将发出 <constant>E_WARNING</constant>
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级别的错误消息（在 PHP 8.0.0 之前为 <constant>E_NOTICE</constant> 级别），并且结果为 &null;。
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</para>
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<para>
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尝试解包标量会将所有变量赋值为 &null;。尝试解包未实现 ArrayAccess 的对象会导致严重错误。
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</para>
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</note>
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</refsect1>
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<refsect1 role="parameters">
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&reftitle.parameters;
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<para>
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<variablelist>
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<varlistentry>
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<term><parameter>var</parameter></term>
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<listitem>
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<para>
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一个变量。
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<variablelist>
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<varlistentry>
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<term><parameter>vars</parameter></term>
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<listitem>
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<para>
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更多变量。
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect1>
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<refsect1 role="returnvalues">
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&reftitle.returnvalues;
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<para>
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返回指定的数组。
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</para>
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</refsect1>
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<refsect1 role="changelog">
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&reftitle.changelog;
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<para>
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<informaltable>
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<tgroup cols="2">
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<thead>
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<row>
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<entry>&Version;</entry>
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<entry>&Description;</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry>7.3.0</entry>
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<entry>
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支持在数组解构时传引用。
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</entry>
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</row>
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<row>
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<entry>7.1.0</entry>
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<entry>
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现在可以指定 <function>list</function> 中的键。
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这就可以解构非数字键或者无顺序的数组。
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</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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</refsect1>
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<refsect1 role="examples">
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&reftitle.examples;
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<para>
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<example>
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<title><function>list</function> 示例</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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$info = array('coffee', 'brown', 'caffeine');
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// 列出所有变量
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list($drink, $color, $power) = $info;
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echo "$drink is $color and $power makes it special.\n";
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// 列出他们的其中一个
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list($drink, , $power) = $info;
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echo "$drink has $power.\n";
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// 或者让我们跳到仅第三个
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list( , , $power) = $info;
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echo "I need $power!\n";
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// list() 不能对字符串起作用
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list($bar) = "abcde";
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var_dump($bar); // NULL
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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<example>
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<title>使用 <function>list</function> 的示例</title>
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<programlisting role="php" annotations="non-interactive">
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<![CDATA[
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<?php
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$result = $pdo->query("SELECT id, name FROM employees");
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while (list($id, $name) = $result->fetch(PDO::FETCH_NUM)) {
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echo "id: $id, name: $name\n";
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}
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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<example>
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<title>使用嵌套的 <function>list</function></title>
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<programlisting role="php">
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<![CDATA[
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<?php
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list($a, list($b, $c)) = array(1, array(2, 3));
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var_dump($a, $b, $c);
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?>
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]]>
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</programlisting>
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&example.outputs;
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<screen>
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<![CDATA[
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int(1)
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int(2)
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int(3)
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]]>
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</screen>
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</example>
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</para>
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<para>
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<function>list</function> 使用 array 索引的顺序和它何时定义无关。
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</para>
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<para>
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<example>
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<title><function>list</function> 和索引顺序定义</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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$foo = array(2 => 'a', 'foo' => 'b', 0 => 'c');
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$foo[1] = 'd';
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list($x, $y, $z) = $foo;
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var_dump($foo, $x, $y, $z);
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]]>
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</programlisting>
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<para>
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得到以下输出（注意比较 <function>list</function> 所写的元素顺序）：
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</para>
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<screen>
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<![CDATA[
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array(4) {
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[2]=>
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string(1) "a"
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["foo"]=>
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string(1) "b"
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[0]=>
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string(1) "c"
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[1]=>
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string(1) "d"
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}
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string(1) "c"
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string(1) "d"
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string(1) "a"
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]]>
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</screen>
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</example>
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</para>
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<para>
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<example>
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<title>带键的 <function>list</function></title>
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<simpara>
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从 PHP 7.1.0 开始，<function>list</function> 可以包含显式的键，可赋值到任意表达式。
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可以混合使用数字和字符串键。但是不能混合有键和无键不能混用。
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</simpara>
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<programlisting role="php">
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<![CDATA[
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<?php
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$data = [
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["id" => 1, "name" => 'Tom'],
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["id" => 2, "name" => 'Fred'],
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];
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foreach ($data as ["id" => $id, "name" => $name]) {
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echo "id: $id, name: $name\n";
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}
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echo PHP_EOL;
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list(1 => $second, 3 => $fourth) = [1, 2, 3, 4];
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echo "$second, $fourth\n";
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]]>
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</programlisting>
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&example.outputs;
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<screen>
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<![CDATA[
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id: 1, name: Tom
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id: 2, name: Fred
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2, 4
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]]>
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</screen>
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</example>
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</para>
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</refsect1>
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<refsect1 role="seealso">
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&reftitle.seealso;
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<para>
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<simplelist>
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<member><function>each</function></member>
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<member><function>array</function></member>
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<member><function>extract</function></member>
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</simplelist>
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</para>
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</refsect1>
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</refentry>
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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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