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123 lines
8.2 KiB
HTML
123 lines
8.2 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<title>Boolean 布尔类型</title>
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</head>
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<div id="layout">
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<div id="layout-content"><div id="language.types.boolean" class="sect1">
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<h2 class="title">Boolean 布尔类型</h2>
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<p class="simpara">
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这是最简单的类型。<span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span> 表达了真值，可以为 <strong><code>TRUE</code></strong> 或 <strong><code>FALSE</code></strong>。
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</p>
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<div class="sect2" id="language.types.boolean.syntax">
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<h3 class="title">语法</h3>
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<p class="para">
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要指定一个布尔值，使用常量 <strong><code>TRUE</code></strong> 或 <strong><code>FALSE</code></strong>。两个都不区分大小写。
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</p>
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><code><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$foo </span><span style="color: #007700">= </span><span style="color: #0000BB">True</span><span style="color: #007700">; </span><span style="color: #FF8000">// 设置 $foo 为 TRUE<br /></span><span style="color: #0000BB">?></span>
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</span>
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</code></div>
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</div>
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</div>
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<p class="para">
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通常<a href="language.operators.html" class="link">运算符</a>所返回的
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<span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span> 值结果会被传递给<a href="language.control-structures.html" class="link">控制流程</a>。
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</p>
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><code><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">// == 是一个操作符，它检测两个变量是否相等，并返回一个布尔值<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">$action </span><span style="color: #007700">== </span><span style="color: #DD0000">"show_version"</span><span style="color: #007700">) {<br /> echo </span><span style="color: #DD0000">"The version is 1.23"</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #FF8000">// 这样做是不必要的...<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">$show_separators </span><span style="color: #007700">== </span><span style="color: #0000BB">TRUE</span><span style="color: #007700">) {<br /> echo </span><span style="color: #DD0000">"<hr>\n"</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #FF8000">// ...因为可以使用下面这种简单的方式：<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">$show_separators</span><span style="color: #007700">) {<br /> echo </span><span style="color: #DD0000">"<hr>\n"</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
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</span>
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</code></div>
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</div>
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</div>
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</div>
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<div class="sect2" id="language.types.boolean.casting">
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<h3 class="title">转换为布尔值</h3>
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<p class="simpara">
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要明确地将一个值转换成 <span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span>，用 <em>(bool)</em>
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或者 <em>(boolean)</em> 来强制转换。但是很多情况下不需要用强制转换，因为当运算符，函数或者流程控制结构需要一个
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<span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span> 参数时，该值会被自动转换。
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</p>
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<p class="simpara">
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参见<a href="language.types.type-juggling.html" class="link">类型转换的判别</a>。
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</p>
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<p class="para">
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当转换为 <span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span> 时，以下值被认为是 <strong><code>FALSE</code></strong>：
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</p>
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<ul class="itemizedlist">
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<li class="listitem">
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<span class="simpara">
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<a href="language.types.boolean.html" class="link">布尔</a>值 <strong><code>FALSE</code></strong> 本身
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</span>
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</li>
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<li class="listitem">
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<span class="simpara">
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<a href="language.types.integer.html" class="link">整型</a>值 0（零）
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</span>
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</li>
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<li class="listitem">
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<span class="simpara">
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<a href="language.types.float.html" class="link">浮点型</a>值 0.0（零）
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</span>
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</li>
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<li class="listitem">
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<span class="simpara">
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空<a href="language.types.string.html" class="link">字符串</a>，以及<a href="language.types.string.html" class="link">字符串</a> "0"
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</span>
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</li>
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<li class="listitem">
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<span class="simpara">
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不包括任何元素的<a href="language.types.array.html" class="link">数组</a>
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</span>
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</li>
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<li class="listitem">
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<span class="simpara">
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特殊类型 <a href="language.types.null.html" class="link">NULL</a>（包括尚未赋值的变量）
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</span>
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</li>
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<li class="listitem">
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<span class="simpara">
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从空标记生成的 <a href="ref.simplexml.html" class="link">SimpleXML</a> 对象
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</span>
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</li>
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</ul>
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<p class="para">
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所有其它值都被认为是 <strong><code>TRUE</code></strong>（包括任何<a href="language.types.resource.html" class="link">资源</a> 和 <strong><code>NAN</code></strong>）。
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</p>
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<div class="warning"><strong class="warning">Warning</strong>
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<p class="simpara">
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<em>-1</em> 和其它非零值（不论正负）一样，被认为是 <strong><code>TRUE</code></strong>！
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</p>
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</div>
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><code><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />var_dump</span><span style="color: #007700">((bool) </span><span style="color: #DD0000">""</span><span style="color: #007700">); </span><span style="color: #FF8000">// bool(false)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) </span><span style="color: #0000BB">1</span><span style="color: #007700">); </span><span style="color: #FF8000">// bool(true)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) -</span><span style="color: #0000BB">2</span><span style="color: #007700">); </span><span style="color: #FF8000">// bool(true)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) </span><span style="color: #DD0000">"foo"</span><span style="color: #007700">); </span><span style="color: #FF8000">// bool(true)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) </span><span style="color: #0000BB">2.3e5</span><span style="color: #007700">); </span><span style="color: #FF8000">// bool(true)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) array(</span><span style="color: #0000BB">12</span><span style="color: #007700">)); </span><span style="color: #FF8000">// bool(true)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) array()); </span><span style="color: #FF8000">// bool(false)<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((bool) </span><span style="color: #DD0000">"false"</span><span style="color: #007700">); </span><span style="color: #FF8000">// bool(true)<br /></span><span style="color: #0000BB">?></span>
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</span>
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</code></div>
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</div>
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</div>
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</div>
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</div></div></div></body></html> |