The difference between isset (variable) and direct judgment variable in PHP

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 ); //1  echo  " ; 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 array is different from map, if there is no key and you go to the value will raise an error instead of returning null, so to have a corresponding judgment, first judge the value of  if  (isset  ( $arr  [ ' 2 ' ]) {//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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