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https://github.com/php/doc-zh.git
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92 lines
2.7 KiB
XML
92 lines
2.7 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<!-- $Author$ -->
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<!-- EN-Revision: 7104ee97ced1768a3231588dfc0bc0d7eb1117ad Maintainer: dallas Status: ready -->
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<sect1 xml:id="control-structures.if" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
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<title>if</title>
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<?phpdoc print-version-for="if"?>
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<para>
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<literal>if</literal> 结构是很多语言包括 PHP
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在内最重要的特性之一，它允许按照条件执行代码片段。PHP 的
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<literal>if</literal> 结构和 C 语言相似：
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<informalexample>
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<programlisting>
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<![CDATA[
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<?php
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if (expr)
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statement
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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<simpara>
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如同在<link
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linkend="language.expressions">表达式</link>一章中定义的，<replaceable>expr</replaceable>
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按照布尔求值。如果 <replaceable>expr</replaceable>
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的值为 &true;，PHP 将执行 <replaceable>statement</replaceable>，如果值为
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&false; ——将忽略 <replaceable>statement</replaceable>。有关哪些值被视为
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&false; 的更多信息参见<link
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linkend="language.types.boolean.casting">转换为布尔值</link>一节。
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</simpara>
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<para>
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如果 <varname>$a</varname> 大于 <varname>$b</varname>，则以下例子将显示
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<computeroutput>a is bigger than b</computeroutput>：
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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<?php
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if ($a > $b)
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echo "a is bigger than b";
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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<para>
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经常需要按照条件执行不止一条语句，当然并不需要给每条语句都加上一个
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<literal>if</literal> 子句。可以将这些语句放入语句组中。例如，如果
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<varname>$a</varname> 大于 <varname>$b</varname>，以下代码将显示
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<computeroutput>a is bigger than b</computeroutput> 并且将
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<varname>$a</varname> 的值赋给 <varname>$b</varname>：
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<informalexample>
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<programlisting role="php">
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<![CDATA[
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<?php
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if ($a > $b) {
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echo "a is bigger than b";
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$b = $a;
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}
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?>
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]]>
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</programlisting>
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</informalexample>
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</para>
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<simpara>
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<literal>if</literal> 语句可以无限层地嵌套在其它
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<literal>if</literal> 语句中，这给程序的不同部分的条件执行提供了充分的弹性。
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</simpara>
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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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