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70 lines
2.5 KiB
HTML
70 lines
2.5 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>NULL</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.null" class="sect1">
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<h2 class="title">NULL</h2>
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<p class="para">
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特殊的 <strong><code>NULL</code></strong> 值表示一个变量没有值。<span class="type"><a href="language.types.null.html" class="type NULL">NULL</a></span> 类型唯一可能的值就是 <strong><code>NULL</code></strong>。
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</p>
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<p class="para">
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在下列情况下一个变量被认为是 <strong><code>NULL</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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<p class="para">
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被赋值为 <strong><code>NULL</code></strong>。
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</p>
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</li>
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<li class="listitem">
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<p class="para">
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尚未被赋值。
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</p>
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</li>
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<li class="listitem">
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<p class="para">
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被 <span class="function"><a href="unset.html" class="function">unset()</a></span>。
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</p>
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</li>
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</ul>
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<div class="sect2" id="language.types.null.syntax">
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<h3 class="title">语法</h3>
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<p class="para">
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<strong><code>NULL</code></strong> 类型只有一个值，就是不区分大小写的常量 <strong><code>NULL</code></strong>。
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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 />$var </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">; <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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参见 <span class="function"><a href="is_null.html" class="function">is_null()</a></span> 和 <span class="function"><a href="unset.html" class="function">unset()</a></span>。
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</p>
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</div>
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<div class="sect2" id="language.types.null.casting">
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<h3 class="title">转换到 <em>NULL</em></h3>
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<p class="para">
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使用 <em>(unset) $var</em> 将一个变量转换为 <span class="type"><a href="language.types.null.html" class="type null">null</a></span>
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将<em class="emphasis">不会</em>删除该变量或 unset 其值。仅是返回 <strong><code>NULL</code></strong> 值而已。
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</p>
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</div>
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</div></div></div></body></html> |