Description:
------------
I want to check, if a variable is not set. If the variable is not set, i want to 
check, if this variable has a special value. There is no problem with "or", but 
with "xor" outputs a notice.

Test script:
---------------
<?
session_start();

if(!isset($_SESSION['foo']) xor $_SESSION['foo'] == 1) echo "foo";
if(!isset($bar) xor $bar== 1) echo "bar";

if(!isset($_SESSION['foo']) or $_SESSION['foo'] == 1) echo "foo";
if(!isset($bar) or $bar== 1) echo "bar";

?>

Expected result:
----------------
foobarfoobar

Actual result:
--------------
Notice: Undefined index: foo in C:\xampp\htdocs\feredoras\testing.php on line 4
foo
Notice: Undefined variable: bar in C:\xampp\htdocs\feredoras\testing.php on line 5
barfoobar