WP-Admin login errors

I have installed a custom built template on my site and have got these errors when I try to log in:

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\folios\201142\blog\wp-content\themes\mytemplate\functions.php:11) in C:\xampp\htdocs\folios\201142\blog\wp-login.php on line 354

and

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\folios\201142\blog\wp-content\themes\mytemplate\functions.php:11) in C:\xampp\htdocs\folios\201142\blog\wp-login.php on line 366

Don’t even get me started when I log out, it tells me I have errors on

Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\folios\201142\blog\wp-content\themes\mytemplate\functions.php:11) in C:\xampp\htdocs\folios\201142\blog\wp-includes\pluggable.php on line 897

amongst other lines in the same pluggable.php file. Is it a template issue or a WP-Admin issue? It calls the functions.php file, here are the contents of the file:

<?php
if ( function_exists('register_sidebar') )
register_sidebar(array(
'before_widget' => '',
'after_widget' => '',
'before_title' => '<h4>',
'after_title' => '</h4>',
));
?>

<?php // for sidebar.php if needed (remove the // below the if() statement) ?>
<?php // if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar() ) : endif; ?>

Any solutions?