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201 lines
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201 lines
10 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>检查一个变量是否为空</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.empty" class="refentry">
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<div class="refnamediv">
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<h1 class="refname">empty</h1>
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<p class="verinfo">(PHP 4, PHP 5, PHP 7)</p><p class="refpurpose"><span class="refname">empty</span> — <span class="dc-title">检查一个变量是否为空</span></p>
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</div>
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<div class="refsect1 description" id="refsect1-function.empty-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>empty</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">$var</code></span>
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) : <span class="type">bool</span></div>
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<p class="para rdfs-comment">
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判断一个变量是否被认为是空的。当一个变量并不存在，或者它的值等同于<strong><code>FALSE</code></strong>，那么它会被认为不存在。如果变量不存在的话，<span class="function"><strong>empty()</strong></span>并不会产生警告。
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</p>
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</div>
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<div class="refsect1 parameters" id="refsect1-function.empty-parameters">
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<h3 class="title">参数</h3>
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<p class="para">
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<dl>
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<dt>
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<code class="parameter">var</code></dt>
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<dd>
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<p class="para">
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待检查的变量
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</p>
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<blockquote class="note"><p><strong class="note">Note</strong>:
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<p class="para">
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在 PHP 5.5 之前，<span class="function"><strong>empty()</strong></span> 仅支持变量；任何其他东西将会导致一个解析错误。换言之，下列代码不会生效： <strong class="command">empty(trim($name))</strong>。
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作为替代，应该使用<strong class="command">trim($name) == false</strong>.
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</p>
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</p></blockquote>
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<p class="para">
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没有警告会产生，哪怕变量并不存在。
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这意味着 <span class="function"><strong>empty()</strong></span> 本质上与 <strong class="command">!isset($var) || $var == false</strong> 等价。
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</p>
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</dd>
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</dl>
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</p>
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</div>
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<div class="refsect1 returnvalues" id="refsect1-function.empty-returnvalues">
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<h3 class="title">返回值</h3>
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<p class="para">
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当<code class="parameter">var</code>存在，并且是一个非空非零的值时返回 <strong><code>FALSE</code></strong>
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否则返回 <strong><code>TRUE</code></strong>.
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</p>
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<p class="para">
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以下的东西被认为是空的：
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<ul class="simplelist">
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<li class="member"><em>""</em> (空字符串)</li>
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<li class="member"><em>0</em> (作为整数的0)</li>
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<li class="member"><em>0.0</em> (作为浮点数的0)</li>
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<li class="member"><em>"0"</em> (作为字符串的0)</li>
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<li class="member"><strong><code>NULL</code></strong></li>
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<li class="member"><strong><code>FALSE</code></strong></li>
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<li class="member"><em>array()</em> (一个空数组)</li>
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<li class="member"><em>$var;</em> (一个声明了，但是没有值的变量)</li>
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</ul>
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</p>
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</div>
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<div class="refsect1 changelog" id="refsect1-function.empty-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>5.5.0</td>
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<td>
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<p class="para">
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<span class="function"><strong>empty()</strong></span> 现在支持表达式了，而不仅仅是变量。
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</p>
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</td>
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</tr>
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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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检查非数字的字符串偏移量会返回 <strong><code>TRUE</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.empty-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-6358">
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<p><strong>Example #1
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一个简单的 <span class="function"><strong>empty()</strong></span> 与 <span class="function"><a href="isset.html" class="function">isset()</a></span> 的比较。
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</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 />$var </span><span style="color: #007700">= </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /><br /></span><span style="color: #FF8000">// Evaluates to true because $var is empty<br /></span><span style="color: #007700">if (empty(</span><span style="color: #0000BB">$var</span><span style="color: #007700">)) {<br /> echo </span><span style="color: #DD0000">'$var is either 0, empty, or not set at all'</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #FF8000">// Evaluates as true because $var is set<br /></span><span style="color: #007700">if (isset(</span><span style="color: #0000BB">$var</span><span style="color: #007700">)) {<br /> echo </span><span style="color: #DD0000">'$var is set even though it is empty'</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
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</span>
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</pre></div>
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</div>
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</div>
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</p>
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<div class="example" id="example-6359">
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<p><strong>Example #2 在字符串偏移量上使用<span class="function"><strong>empty()</strong></span></strong></p>
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<div class="example-contents"><p>
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PHP 5.4 修改了当传入的是字符串偏移量时， <span class="function"><strong>empty()</strong></span> 的行为
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</p></div>
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<div class="example-contents">
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<div class="phpcode"><pre><span style="color: #000000">
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<span style="color: #0000BB"><?php<br />$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">(empty(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'some_key'</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(empty(</span><span style="color: #0000BB">$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">(empty(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'0'</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(empty(</span><span style="color: #0000BB">$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">(empty(</span><span style="color: #0000BB">$expected_array_got_string</span><span style="color: #007700">[</span><span style="color: #DD0000">'0.5'</span><span style="color: #007700">]));<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(empty(</span><span style="color: #0000BB">$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(false)
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bool(false)
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bool(false)
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bool(false)
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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 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(true)
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bool(false)
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bool(false)
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bool(false)
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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>
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</div>
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<div class="refsect1 notes" id="refsect1-function.empty-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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当对一个不可见的对象属性使用 <span class="function"><strong>empty()</strong></span> 时，
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<a href="language.oop5.overloading.html#object.isset" class="link">__isset()</a>
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方法如果存在的话，它将会被调用。
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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.empty-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"><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="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="count.html" class="function" rel="rdfs-seeAlso">count()</a> - 计算数组中的单元数目，或对象中的属性个数</span></li>
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<li class="member"><span class="function"><a href="strlen.html" class="function" rel="rdfs-seeAlso">strlen()</a> - 获取字符串长度</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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</ul>
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</p>
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</div>
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</div></div></div></body></html> |