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218 lines
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218 lines
16 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>检测变量是否已设置并且非 NULL</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.isset" class="refentry">
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<div class="refnamediv">
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<h1 class="refname">isset</h1>
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<p class="verinfo">(PHP 4, PHP 5, PHP 7)</p><p class="refpurpose"><span class="refname">isset</span> — <span class="dc-title">检测变量是否已设置并且非 <strong><code>NULL</code></strong></span></p>
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</div>
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<div class="refsect1 description" id="refsect1-function.isset-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>isset</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">bool</span></div>
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<p class="para rdfs-comment">
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检测变量是否设置，并且不是 <strong><code>NULL</code></strong>。
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</p>
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<p class="para">
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如果已经使用 <span class="function"><a href="unset.html" class="function">unset()</a></span>
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释放了一个变量之后，它将不再是
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<span class="function"><strong>isset()</strong></span>。若使用 <span class="function"><strong>isset()</strong></span>
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测试一个被设置成 <strong><code>NULL</code></strong> 的变量，将返回 <strong><code>FALSE</code></strong>。同时要注意的是 null 字符（<em>"\0"</em>）并不等同于
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PHP 的 <strong><code>NULL</code></strong> 常量。
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</p>
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<p class="para">
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如果一次传入多个参数，那么
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<span class="function"><strong>isset()</strong></span> 只有在全部参数都以被设置时返回 <strong><code>TRUE</code></strong> 计算过程从左至右，中途遇到没有设置的变量时就会立即停止。
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</p>
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</div>
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<div class="refsect1 parameters" id="refsect1-function.isset-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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要检查的变量。
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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.isset-returnvalues">
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<h3 class="title">返回值</h3>
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<p class="para">
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如果 <code class="parameter">var</code> 存在并且值不是 <strong><code>NULL</code></strong> 则返回 <strong><code>TRUE</code></strong>，否则返回 <strong><code>FALSE</code></strong>。
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</p>
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</div>
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<div class="refsect1 changelog" id="refsect1-function.isset-changelog">
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<h3 class="title">更新日志</h3>
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<p class="para">
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<table class="doctable informaltable">
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<thead>
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<tr>
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<th>版本</th>
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<th>说明</th>
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</tr>
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</thead>
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<tbody class="tbody">
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<tr>
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<td>5.4.0</td>
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<td>
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<p class="para">
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检查字符的非数字偏移量将会返回 <strong><code>FALSE</code></strong>。
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</p>
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</td>
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</tr>
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</tbody>
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</table>
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</p>
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</div>
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<div class="refsect1 examples" id="refsect1-function.isset-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-6365">
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<p><strong>Example #1 <span class="function"><strong>isset()</strong></span> 例子</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 /><br />$var </span><span style="color: #007700">= </span><span style="color: #DD0000">''</span><span style="color: #007700">;<br /><br /></span><span style="color: #FF8000">// 结果为 TRUE，所以后边的文本将被打印出来。<br /></span><span style="color: #007700">if (isset(</span><span style="color: #0000BB">$var</span><span style="color: #007700">)) {<br /> echo </span><span style="color: #DD0000">"This var is set so I will print."</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #FF8000">// 在后边的例子中，我们将使用 var_dump 输出 isset() 的返回值。<br />// the return value of isset().<br /><br /></span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #DD0000">"test"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$b </span><span style="color: #007700">= </span><span style="color: #DD0000">"anothertest"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">)); </span><span style="color: #FF8000">// TRUE<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">, </span><span style="color: #0000BB">$b</span><span style="color: #007700">)); </span><span style="color: #FF8000">// TRUE<br /><br /></span><span style="color: #007700">unset (</span><span style="color: #0000BB">$a</span><span style="color: #007700">);<br /><br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">)); </span><span style="color: #FF8000">// FALSE<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">, </span><span style="color: #0000BB">$b</span><span style="color: #007700">)); </span><span style="color: #FF8000">// FALSE<br /><br /></span><span style="color: #0000BB">$foo </span><span style="color: #007700">= </span><span style="color: #0000BB">NULL</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$foo</span><span style="color: #007700">)); </span><span style="color: #FF8000">// FALSE<br /><br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<p class="para">
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这对于数组中的元素也同样有效：
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<div class="informalexample">
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br /><br />$a </span><span style="color: #007700">= array (</span><span style="color: #DD0000">'test' </span><span style="color: #007700">=> </span><span style="color: #0000BB">1</span><span style="color: #007700">, </span><span style="color: #DD0000">'hello' </span><span style="color: #007700">=> </span><span style="color: #0000BB">NULL</span><span style="color: #007700">, </span><span style="color: #DD0000">'pie' </span><span style="color: #007700">=> array(</span><span style="color: #DD0000">'a' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'apple'</span><span style="color: #007700">));<br /><br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">[</span><span style="color: #DD0000">'test'</span><span style="color: #007700">])); </span><span style="color: #FF8000">// TRUE<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">[</span><span style="color: #DD0000">'foo'</span><span style="color: #007700">])); </span><span style="color: #FF8000">// FALSE<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">[</span><span style="color: #DD0000">'hello'</span><span style="color: #007700">])); </span><span style="color: #FF8000">// FALSE<br /><br />// 键 'hello' 的值等于 NULL，所以被认为是未置值的。<br />// 如果想检测 NULL 键值，可以试试下边的方法。 <br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">array_key_exists</span><span style="color: #007700">(</span><span style="color: #DD0000">'hello'</span><span style="color: #007700">, </span><span style="color: #0000BB">$a</span><span style="color: #007700">)); </span><span style="color: #FF8000">// TRUE<br /><br />// Checking deeper array values<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">[</span><span style="color: #DD0000">'pie'</span><span style="color: #007700">][</span><span style="color: #DD0000">'a'</span><span style="color: #007700">])); </span><span style="color: #FF8000">// TRUE<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">[</span><span style="color: #DD0000">'pie'</span><span style="color: #007700">][</span><span style="color: #DD0000">'b'</span><span style="color: #007700">])); </span><span style="color: #FF8000">// FALSE<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$a</span><span style="color: #007700">[</span><span style="color: #DD0000">'cake'</span><span style="color: #007700">][</span><span style="color: #DD0000">'a'</span><span style="color: #007700">][</span><span style="color: #DD0000">'b'</span><span style="color: #007700">])); </span><span style="color: #FF8000">// FALSE<br /><br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<div class="example" id="example-6366">
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<p><strong>Example #2 在字符串位移中使用 <span class="function"><strong>isset()</strong></span></strong></p>
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<div class="example-contents"><p>
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PHP 5.4 改变了传入字符串位移时 <span class="function"><strong>isset()</strong></span> 的行为。
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</p></div>
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$expected_array_got_string </span><span style="color: #007700">= </span><span style="color: #DD0000">'somestring'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'some_key'</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #0000BB">0</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'0'</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #0000BB">0.5</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'0.5'</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(isset(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'0 Mostel'</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>以上例程在PHP 5.3中的输出：</p></div>
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<div class="example-contents screen">
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<div class="cdata"><pre>
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bool(true)
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bool(true)
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bool(true)
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bool(true)
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bool(true)
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bool(true)
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</pre></div>
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</div>
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<div class="example-contents"><p>以上例程在PHP 5.4中的输出：</p></div>
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<div class="example-contents screen">
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<div class="cdata"><pre>
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bool(false)
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bool(true)
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bool(true)
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bool(true)
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bool(false)
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bool(false)
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</pre></div>
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</div>
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</div>
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</div>
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<div class="refsect1 notes" id="refsect1-function.isset-notes">
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<h3 class="title">注释</h3>
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<div class="warning"><strong class="warning">Warning</strong>
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<p class="para">
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<span class="function"><strong>isset()</strong></span>
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只能用于变量，因为传递任何其它参数都将造成解析错误。若想检测<a href="language.constants.html" class="link">常量</a>是否已设置，可使用
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<span class="function"><a href="defined.html" class="function">defined()</a></span> 函数。
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</p>
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</div>
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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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如果使用 <span class="function"><strong>isset()</strong></span> 来检查对象无法访问的属性，如果 <a href="language.oop5.overloading.html#object.isset" class="link">__isset()</a>
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方法已经定义则会调用这个重载方法。
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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.isset-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="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.isset" class="link">__isset()</a></li>
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<li class="member"><span class="function"><a href="unset.html" class="function" rel="rdfs-seeAlso">unset()</a> - 释放给定的变量</span></li>
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<li class="member"><span class="function"><a href="defined.html" class="function" rel="rdfs-seeAlso">defined()</a> - 检查某个名称的常量是否存在</span></li>
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<li class="member"><a href="types.comparisons.html" class="link">the type comparison tables</a></li>
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<li class="member"><span class="function"><a href="array_key_exists.html" class="function" rel="rdfs-seeAlso">array_key_exists()</a> - 检查数组里是否有指定的键名或索引</span></li>
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<li class="member"><span class="function"><a href="is_null.html" class="function" rel="rdfs-seeAlso">is_null()</a> - 检测变量是否为 NULL</span></li>
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<li class="member">错误控制
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<a href="language.operators.errorcontrol.html" class="link">@</a>
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运算符。</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> |