PHP中isset(变量)跟直接判断变量的区别

PHP中isset(变量)和直接判断变量的区别

在变量为空字符串和布尔值false以及数值0时，isset全部返回true，直接判断全部返回false：

<code class=" hljs php"><span class="hljs-variable">$empty</span>=<span class="hljs-string">''</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-keyword">isset</span>(<span class="hljs-variable">$empty</span>)?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//1</span><span class="hljs-keyword">echo</span> <span class="hljs-string">'<br/>'</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-variable">$empty</span>?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//0</span></code>

<code class=" hljs php"><span class="hljs-variable">$empty</span>=<span class="hljs-keyword">false</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-keyword">isset</span>(<span class="hljs-variable">$empty</span>)?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//1</span><span class="hljs-keyword">echo</span> <span class="hljs-string">'<br/>'</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-variable">$empty</span>?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//0</span></code>

<code class=" hljs php"><span class="hljs-variable">$empty</span>=<span class="hljs-number">0</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-keyword">isset</span>(<span class="hljs-variable">$empty</span>)?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//1</span><span class="hljs-keyword">echo</span> <span class="hljs-string">'<br/>'</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-variable">$empty</span>?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//0</span></code>

只有在变量为null时，它们才是没有区别的：

<code class=" hljs php"><span class="hljs-variable">$empty</span>=<span class="hljs-keyword">null</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-keyword">isset</span>(<span class="hljs-variable">$empty</span>)?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//0</span><span class="hljs-keyword">echo</span> <span class="hljs-string">'<br/>'</span>;<span class="hljs-keyword">echo</span> (<span class="hljs-variable">$empty</span>?<span class="hljs-string">'1'</span>:<span class="hljs-number">0</span>);<span class="hljs-comment">//0</span></code>

虽然isset在判断变量没什么作用，但是在判断数组有无一个变量却很好用：

<code class=" hljs php"><span class="hljs-variable">$arr</span>=<span class="hljs-keyword">array</span>(<span class="hljs-string">'1'</span>=><span class="hljs-number">1</span>,<span class="hljs-string">'3'</span>=><span class="hljs-number">3</span>);<span class="hljs-variable">$temp</span>=<span class="hljs-keyword">null</span>;<span class="hljs-comment">//PS:php中数组不同于map，如果没有key而你去取值会引发错误而不是返回null，所以要有对应的判断，先判断再取值</span><span class="hljs-keyword">if</span> (<span class="hljs-keyword">isset</span>(<span class="hljs-variable">$arr</span>[<span class="hljs-string">'2'</span>])) {<span class="hljs-comment">//效果等同于array_key_exists('2', $arr)，但相对代码短一点</span>    <span class="hljs-variable">$temp</span>=<span class="hljs-variable">$arr</span>[<span class="hljs-string">'2'</span>];}<span class="hljs-keyword">echo</span> <span class="hljs-variable">$temp</span>;</code>

还有对isset不了解的可以去官网看一下就明白了：
http://php.net/manual/zh/function.isset.php

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