PHP unassociated variables sitting in code doing nothing -

i came across this question features following code piece:

<?php    class goat{      public $goat_id;      public $goat_name;      public $hashed_password;   ....    public static function authenticate($username, $password){          $is_auth; // line         if(!empty($result)){             $user = array_shift($result);             $is_auth = pass_auth::verify_password($password, $user- >hashed_password);          }           if($is_auth) {              return $user;          }       ....     }  } 

my question comes marked line inside authenticate function. not example -but in general- point of $is_auth; declaration, sitting there doing nothing?

i have tried find answer before posting here if layout/behaviour has specific name don't know it.

i have made sandbox variations , surprised find none of them produce errors, below code error free:

<?php    error_reporting(e_all);     $small = "tree";    //$small = false; //swapping var value types makes no difference.    $small;     $sdgdfghdgd;  

so questions are:

• what name unassociated variable usage?
• is there point this?
  • if so; what?
• why doesn't produce error or warning (i'm not saying should, necessarily, curious)?

$is_auth; 

is valid php expression. it's value value of $is_auth , doesn't have effect.

the php compiler smart enough detect expression useless (because it's value not used , doesn't have side effect) , doesn't generate opcode it.

if generating code, code should trigger e_notice because variable not defined yet.

the situation changes when expression involves array. if, conceptually, expression $a['foo']; equivalent $is_auth;, $a['foo']; not optimized out compiler. evaluated , triggers expected e_notice when $a not defined or doesn't have key 'foo'.