$_REQUEST with variables

Hi I have 2 windows.
in the first i have a form and 2 textfields and one variable. In the
second Window i want to display the data -> the variable "c" is the
most important but I get the error: Undefined index: c (a and b are
working fine)!

so how can i pass this variable?

WINDOW1
*************
<form action="window2.php" method="post">
<?php
$c = 'some text';
?>
<input type="Text" name="a">
<input type="Text" name="b">
<input type="submit" name="Submit" value="submit">
</form>

WINDOW2
*************

<?php
echo $_REQUEST['a'];
echo $_REQUEST['b'];
echo $_REQUEST['c'];
?>




Notice: Undefined index: c in c:\inetpub\wwwroot\fachstelle\window2.php
on line 22

.