Errors when Posting

When I try making a new post, the following appears in the Admin CP below Word Count:

Cannot modify header information – headers already sent by (output started at /home/anonymod/public_html/guardtunnel.com/news/wp-content/themes/plainscape/functions.php:18) in /home/anonymod/public_html/guardtunnel.com/news/wp-includes/classes.php on line 1586

Then, when I press Submit, the entire page goes blank except for the PHP error:

Warning: Cannot modify header information – headers already sent by (output started at /home/anonymod/public_html/guardtunnel.com/news/wp-content/themes/plainscape/functions.php:18) in /home/anonymod/public_html/guardtunnel.com/news/wp-includes/pluggable.php on line 865

The only thing that my functions has is

<?php
if ( function_exists(‘register_sidebar’) )
register_sidebar(array(
‘before_widget’ => ‘<li id=”%1$s” class=”widget %2$s”>’,
‘after_widget’ => ”,
‘before_title’ => ‘<h2 class=”widgettitle”>’,
‘after_title’ => ‘</h2>’,
));

add_filter(‘comments_template’, ‘legacy_comments’);
function legacy_comments($file) {
if(!function_exists(‘wp_list_comments’)) $file = TEMPLATEPATH . ‘/legacy.comments.php’;
return $file;
}

?>

I can’t figure out what’s wrong…