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237 lines
14 KiB
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<!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>释放给定的变量</title>
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</head>
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<body class="docs"><div id="layout">
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<div id="layout-content"><div id="function.unset" class="refentry">
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<div class="refnamediv">
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<h1 class="refname">unset</h1>
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<p class="verinfo">(PHP 4, PHP 5, PHP 7)</p><p class="refpurpose"><span class="refname">unset</span> — <span class="dc-title">释放给定的变量</span></p>
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</div>
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<div class="refsect1 description" id="refsect1-function.unset-description">
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<h3 class="title">说明</h3>
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<div class="methodsynopsis dc-description">
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<span class="methodname"><strong>unset</strong></span>
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( <span class="methodparam"><span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <code class="parameter">$var</code></span>
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[, <span class="methodparam"><span class="type"><a href="language.pseudo-types.html#language.types.mixed" class="type mixed">mixed</a></span> <code class="parameter">$...</code></span>
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] ) : <span class="type"><span class="type void">void</span></span></div>
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<p class="para rdfs-comment">
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<span class="function"><strong>unset()</strong></span> 销毁指定的变量。
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</p>
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<p class="para">
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<span class="function"><strong>unset()</strong></span> 在函数中的行为会依赖于想要销毁的变量的类型而有所不同。
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</p>
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<p class="para">
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如果在函数中 <span class="function"><strong>unset()</strong></span>
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一个全局变量，则只是局部变量被销毁，而在调用环境中的变量将保持调用
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<span class="function"><strong>unset()</strong></span> 之前一样的值。
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">destroy_foo</span><span style="color: #007700">() {<br /> global </span><span style="color: #0000BB">$foo</span><span style="color: #007700">;<br /> unset(</span><span style="color: #0000BB">$foo</span><span style="color: #007700">);<br />}<br /><br /></span><span style="color: #0000BB">$foo </span><span style="color: #007700">= </span><span style="color: #DD0000">'bar'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">destroy_foo</span><span style="color: #007700">();<br />echo </span><span style="color: #0000BB">$foo</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">以上例程会输出：</p>
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<p class="para">
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<div class="informalexample">
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<div class="example-contents screen">
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<div class="cdata"><pre>
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bar
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">
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如果您想在函数中 <span class="function"><strong>unset()</strong></span>
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一个全局变量，可使用 <var class="varname"><var class="varname"><a href="reserved.variables.globals.html" class="classname">$GLOBALS</a></var></var>
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数组来实现：
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">foo</span><span style="color: #007700">() <br />{<br /> unset(</span><span style="color: #0000BB">$GLOBALS</span><span style="color: #007700">[</span><span style="color: #DD0000">'bar'</span><span style="color: #007700">]);<br />}<br /><br /></span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #DD0000">"something"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">
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如果在函数中 <span class="function"><strong>unset()</strong></span>
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一个通过引用传递的变量，则只是局部变量被销毁，而在调用环境中的变量将保持调用
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<span class="function"><strong>unset()</strong></span> 之前一样的值。
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">foo</span><span style="color: #007700">(&</span><span style="color: #0000BB">$bar</span><span style="color: #007700">) {<br /> unset(</span><span style="color: #0000BB">$bar</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #DD0000">"blah"</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #DD0000">'something'</span><span style="color: #007700">;<br />echo </span><span style="color: #DD0000">"</span><span style="color: #0000BB">$bar</span><span style="color: #DD0000">\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">foo</span><span style="color: #007700">(</span><span style="color: #0000BB">$bar</span><span style="color: #007700">);<br />echo </span><span style="color: #DD0000">"</span><span style="color: #0000BB">$bar</span><span style="color: #DD0000">\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">以上例程会输出：</p>
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<p class="para">
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<div class="informalexample">
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<div class="example-contents screen">
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<div class="cdata"><pre>
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something
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something
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">
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如果在函数中 <span class="function"><strong>unset()</strong></span>
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一个静态变量，那么在函数内部此静态变量将被销毁。但是，当再次调用此函数时，此静态变量将被复原为上次被销毁之前的值。
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">foo</span><span style="color: #007700">()<br />{<br /> static </span><span style="color: #0000BB">$bar</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">$bar</span><span style="color: #007700">++;<br /> echo </span><span style="color: #DD0000">"Before unset: </span><span style="color: #0000BB">$bar</span><span style="color: #DD0000">, "</span><span style="color: #007700">;<br /> unset(</span><span style="color: #0000BB">$bar</span><span style="color: #007700">);<br /> </span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #0000BB">23</span><span style="color: #007700">;<br /> echo </span><span style="color: #DD0000">"after unset: </span><span style="color: #0000BB">$bar</span><span style="color: #DD0000">\n"</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">以上例程会输出：</p>
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<p class="para">
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<div class="informalexample">
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<div class="example-contents screen">
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<div class="cdata"><pre>
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Before unset: 1, after unset: 23
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Before unset: 2, after unset: 23
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Before unset: 3, after unset: 23
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</pre></div>
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</div>
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</div>
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</p>
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</div>
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<div class="refsect1 parameters" id="refsect1-function.unset-parameters">
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<h3 class="title">参数</h3>
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<p class="para">
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<dl>
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<dt>
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<code class="parameter">var</code></dt>
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<dd>
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<p class="para">
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要销毁的变量。
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</p>
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</dd>
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<dt>
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<code class="parameter">...</code></dt>
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<dd>
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<p class="para">
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其他变量……
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</p>
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</dd>
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</dl>
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</p>
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</div>
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<div class="refsect1 returnvalues" id="refsect1-function.unset-returnvalues">
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<h3 class="title">返回值</h3>
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<p class="para">
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没有返回值。
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</p>
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</div>
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<div class="refsect1 examples" id="refsect1-function.unset-examples">
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<h3 class="title">范例</h3>
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<p class="para">
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<div class="example" id="example-6373">
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<p><strong>Example #1 <span class="function"><strong>unset()</strong></span> 示例</strong></p>
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">// 销毁单个变量<br /></span><span style="color: #007700">unset (</span><span style="color: #0000BB">$foo</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">// 销毁单个数组元素<br /></span><span style="color: #007700">unset (</span><span style="color: #0000BB">$bar</span><span style="color: #007700">[</span><span style="color: #DD0000">'quux'</span><span style="color: #007700">]);<br /><br /></span><span style="color: #FF8000">// 销毁一个以上的变量<br /></span><span style="color: #007700">unset(</span><span style="color: #0000BB">$foo1</span><span style="color: #007700">, </span><span style="color: #0000BB">$foo2</span><span style="color: #007700">, </span><span style="color: #0000BB">$foo3</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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<div class="example" id="example-6374">
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<p><strong>Example #2 使用 <em>(unset)</em> 类型强制转换</strong></p>
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<div class="example-contents"><p>
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<em>(unset)</em> 类型强制转换常常和函数 <span class="function"><strong>unset()</strong></span> 引起困惑。
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为了完整性，<em>(unset)</em> 是作为一个 <em>NULL</em> 类型的强制转换。它不会改变变量的类型。
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</p></div>
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$name </span><span style="color: #007700">= </span><span style="color: #DD0000">'Felipe'</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">((unset) </span><span style="color: #0000BB">$name</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$name</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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<div class="example-contents"><p>以上例程会输出：</p></div>
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<div class="example-contents screen">
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<div class="cdata"><pre>
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NULL
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string(6) "Felipe"
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</pre></div>
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</div>
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</div>
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</p>
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</div>
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<div class="refsect1 notes" id="refsect1-function.unset-notes">
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<h3 class="title">注释</h3>
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<blockquote class="note"><p><strong class="note">Note</strong>: <span class="simpara">因为是一个语言构造器而不是一个函数，不能被
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<a href="functions.variable_functions.html" class="link">可变函数</a> 调用。
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</span>
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</p></blockquote>
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<blockquote class="note"><p><strong class="note">Note</strong>:
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<p class="para">
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It is possible to unset even object properties visible in current context.
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</p>
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</p></blockquote>
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<blockquote class="note"><p><strong class="note">Note</strong>:
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<p class="para">
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在 PHP 5 之前无法在对象里销毁 <em>$this</em>。
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</p>
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</p></blockquote>
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<blockquote class="note"><p><strong class="note">Note</strong>:
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<p class="para">
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在 <span class="function"><strong>unset()</strong></span> 一个无法访问的对象属性时，如果定义了 <a href="language.oop5.overloading.html#object.unset" class="link">__unset()</a> 则对调用这个重载方法。
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</p>
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</p></blockquote>
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</div>
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<div class="refsect1 seealso" id="refsect1-function.unset-seealso">
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<h3 class="title">参见</h3>
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<p class="para">
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<ul class="simplelist">
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<li class="member"><span class="function"><a href="isset.html" class="function" rel="rdfs-seeAlso">isset()</a> - 检测变量是否已设置并且非 NULL</span></li>
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<li class="member"><span class="function"><a href="empty.html" class="function" rel="rdfs-seeAlso">empty()</a> - 检查一个变量是否为空</span></li>
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<li class="member"><a href="language.oop5.overloading.html#object.unset" class="link">__unset()</a></li>
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<li class="member"><span class="function"><a href="array_splice.html" class="function" rel="rdfs-seeAlso">array_splice()</a> - 去掉数组中的某一部分并用其它值取代</span></li>
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</ul>
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</p>
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</div>
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</div></div></div></body></html> |