Description:
------------
current php no best way check a variable whether be defined, only like below:
if check $a whether be defined
in global varibale:
array_key_exists('a', $GLOBALS);
in function or method:
array_key_exists('a',get_defined_vars())

because isset, empty is not trust, see test script

Test script:
---------------
<?php

function test() {
    
    //defined variable
    $a = null;
    var_dump(isset($a)); //false
    var_dump(is_null($a));//true
    var_dump(empty($a)); //true

    //undefined variable
    var_dump(isset($b));//false
    var_dump(empty($a)); //true
    var_dump(@is_null($c));//true, Notice: Undefined variable: a
    var_dump(get_defined_vars());//$b,$c undefined, $a is defined and value is NULL
    
    //be unset() variable
    $d;
    unset($d);
    var_dump(empty($a)); //true
    var_dump(isset($d));//false
    var_dump(@is_null($d));//true,Notice: Undefined variable: a
    var_dump(get_defined_vars());//$b,$c undefined,$d be unset and defined, $a is defined and value is NULL
    
    //empty string variable
    $e = '';
    var_dump(empty($e)); //true
    var_dump(isset($e)); // true
    var_dump(is_null($e)); //false
    var_dump(get_defined_vars());//$b,$c undefined,$d be unset and undefined, $a is defined and value is NULL, $e is defined and value is empty string
}

test();

Expected result:
----------------
<?php
$a;
var_dump(isset($a)); // is false

Actual result:
--------------
<?php
$a;
var_dump(isset($a)); // is true

?>
or 
<?php
$a;
var_dump(isdefined($a)); //is true