isset

名称:isset

分类:变量处理

所属语言:php

一句话介绍:检查变量是否已设置（声明且不为 NULL）。

实例

检查变量是否为空。还要检查变量是否已设置/声明：

<span class="token php language-php"><span class="token delimiter important"><?php</span>
<span class="token variable">$a</span> <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token comment">// True because $a is set</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">isset</span><span class="token punctuation">(</span><span class="token variable">$a</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">echo</span> <span class="token string double-quoted-string">"Variable 'a' is set.<br>"</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token variable">$b</span> <span class="token operator">=</span> <span class="token constant">null</span><span class="token punctuation">;</span>
<span class="token comment">// False because $b is NULL</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token keyword">isset</span><span class="token punctuation">(</span><span class="token variable">$b</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">echo</span> <span class="token string double-quoted-string">"Variable 'b' is set."</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token delimiter important">?></span></span>

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