getting an "undefined variable" on isset [duplicate] - php

Question

This question already has answers here:
"Notice: Undefined variable", "Notice: Undefined index", "Warning: Undefined array key", and "Notice: Undefined offset" using PHP
(29 answers)
Closed 6 years ago.
So i've got a rather complex bit of code that i'm trying to make work (actually; it works fine on my test server but not so much my live server. so i'm trying to make it rework) -- it keeps telling me that a variable i'm asking it to check with isset is not a defined variable. I'm probably over thinking this whole method and there's probably a simplier way. but i need you guys to bash me over the head with it apparently
include('bootstra.php');
include('includes/parts/head.php');
if(!isset($_SESSION['user']['uid']))
{
echo $html;
include('includes/parts/login.php');
if(isset($_GET['mode']))
{
$file = $_GET['mode'];
$path = "includes/parts/{$file}.php";
if (file_exists($path)) {
require_once($path);
} else {
die("The Page REquested could not be found!");
}
}
else
{
include('includes/parts/frontpage.php');
}
}
else
{
if(!isset($_SESSION['x']) || !isset($_SESSION['y']))
{
if(isset($_GET['x']) && isset($_GET['y']))
{
$_SESSION['x'] = $_GET['x'];
$_SESSION['y'] = $_GET['y'];
}
else
{
$_SESSION['x'] = 0;
$_SESSION['y'] = 0;
}
header("location:index.php?x={$_SESSION['x']}&y={$_SESSION['y']}");
}
else
{
if($_SESSION['x'] != $_GET['x'] || $_SESSION['y'] != $_GET['y'] )
{
header("location:index.php?x={$_SESSION['x']}&y={$_SESSION['y']}");
}
else
{
echo $html;
echo $base->displayMap($_GET['x'], $_GET['y']);
include('includes/context_menu.php');
}
}
}
Here is the exact error:
Notice: Undefined index: x in /home/****/public_html/reddactgame/index.php on line 27

Answer 1

Change it
<pre>
if( !isset($_SESSION['x']) || !isset($_SESSION['y']) )
</pre>
to
<pre>
if( !( isset($_SESSION['x']) && isset($_SESSION['y']) ) )
</pre>

Answer 2

There is no error, that is a Warning...
It is saying that $_SESSION['x'] was never setted, so, when you do that isset it tells you that it was never declared.
Example:
This gives you the same warning
empty($x);
This does not give you warning
$x = 0;
empty($x);
EDIT
I see this is a common question, so here is a better explanation.