Warning when editing pages.

I’m getting this warning when trying to edit my pages name or content:

Warning: Cannot modify header information – headers already sent by (output started at /home/ladyga/public_html/test/wp-content/themes/unknown/functions.php:11) in /home/ladyga/public_html/test/wp-includes/pluggable.php on line 865

this is my site my site and I’ve widgetized my sidebar, so according to all the tutorials out there I had to create a functions.php file with this code on it:

<?php
if ( function_exists('register_sidebar') )
register_sidebar(array(
'before_widget' => '',
'after_widget' => '',
'before_title' => '<h2>',
'after_title' => '</h2>',
));
?>

I did so, in the sidebar.php I added the codes like this:

<div id="sidebar-a">

<div class="padding">

<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar() ) : ?>

			<?php include (TEMPLATEPATH . '/searchform.php'); ?>.       

<h2>Archives</h2>

<ul>

<?php wp_get_archives('type=monthly'); ?>

</ul>	

<h2>Categories</h2>

<ul>

<?php wp_list_categories('show_count=1&title_li='); ?>

</ul>

<?php endif; ?>

		</div>
</div>

Why is my functions.php is affecting other admin files?? I don’t know what I’m doing wrong..