The difference between isset (variable) and direct judgment variables 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); the array in the $temp =null;//ps:php is different from map, and if you do not have a key and you go to the value will throw an error instead of returning null, so to have a corresponding judgment, first determine the value if ( Isset ($arr [' 2 '])) {//The effect is equivalent to Array_key_exists (' 2 ', $arr), but the relative code is a little shorter    $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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