The difference between isset variable and direct judgment variable in PHP

When the variable is an empty string and a Boolean value of false and the value 0 o'clock, isset all returns True, directly judging all return false:

$empty='';echo (isset($empty)?'1':0);//1echo'
';echo ($empty?'1':0);//0

$empty=false;echo (isset($empty)?'1':0);//1echo'
';echo ($empty?'1':0);//0

$empty=0;echo (isset($empty)?'1':0);//1echo'
';echo ($empty?'1':0);//0

They are indistinguishable only when the variables are null:

$empty=null;echo (isset($empty)?'1':0);//0echo'
';echo ($empty?'1':0);//0

Although Isset does not work in judging variables, it is useful to have a variable in the judgment array:

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

And the isset do not know can go to the official website to see the understanding:
http://php.net/manual/zh/function.isset.php

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