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

Copyright Statement: This article is an original article by the blogger and may not be reproduced without the blogger's permission.

The above has introduced the difference between isset variables and direct judgment variables in PHP, including the relevant aspects. I hope it will be helpful to friends who are interested in PHP tutorials.