The difference between isset variables and direct judgment variables in PHP

When the variable is an empty string, a Boolean value of false, and a value of 0, isset all returns true, and direct judgment returns all false:

<code><span>$empty</span>=<span>''</span>;
<span>echo</span> (<span>isset</span>(<span>$empty</span>)?<span>'1'</span>:<span>0</span>);<span>//1</span><span>echo</span><span>'<br/>'</span>;
<span>echo</span> (<span>$empty</span>?<span>'1'</span>:<span>0</span>);<span>//0</span></code>

<code><span>$empty</span>=<span>false</span>;
<span>echo</span> (<span>isset</span>(<span>$empty</span>)?<span>'1'</span>:<span>0</span>);<span>//1</span><span>echo</span><span>'<br/>'</span>;
<span>echo</span> (<span>$empty</span>?<span>'1'</span>:<span>0</span>);<span>//0</span></code>

<code><span>$empty</span>=<span>0</span>;
<span>echo</span> (<span>isset</span>(<span>$empty</span>)?<span>'1'</span>:<span>0</span>);<span>//1</span><span>echo</span><span>'<br/>'</span>;
<span>echo</span> (<span>$empty</span>?<span>'1'</span>:<span>0</span>);<span>//0</span></code>

Only when the variable is null, there is no difference between them:

<code><span>$empty</span>=<span>null</span>;
<span>echo</span> (<span>isset</span>(<span>$empty</span>)?<span>'1'</span>:<span>0</span>);<span>//0</span><span>echo</span><span>'<br/>'</span>;
<span>echo</span> (<span>$empty</span>?<span>'1'</span>:<span>0</span>);<span>//0</span></code>

Although isset It is of no use in judging variables, but it is very useful in judging whether an array has a variable:

<code><span>$arr</span>=<span>array</span>(<span>'1'</span>=><span>1</span>,<span>'3'</span>=><span>3</span>);
<span>$temp</span>=<span>null</span>;
<span>//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>if</span> (<span>isset</span>(<span>$arr</span>[<span>'2'</span>])) {<span>//效果等同于array_key_exists('2', $arr)，但相对代码短一点</span><span>$temp</span>=<span>$arr</span>[<span>'2'</span>];
}
<span>echo</span><span>$temp</span>;</code>

If you don’t know about isset, you can go to the official website and have a look and you will understand:
http://php.net/manual/zh/function.isset.php

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