functions.php in theme causes error in wp-admin

Hi there,

I’ve added a functions.php to my theme with a simple function for displaying authors.

<?php
  // display all users with a link to their user page
  function show_authors() {
      global $wpdb;
      $users = $wpdb->get_col("SELECT * FROM $wpdb->users ORDER BY user_nicename");
      foreach($users as $user) {
          $user = get_userdata($user);
          echo '<li><a href="' . get_author_posts_url($user->ID, $user->user_nicename) . '">' . $user->display_name . '</a></li>';
      }
 } ?>

works fine, however when i went back to login onto my wp-admin interface i got this error

Warning: Cannot modify header information - headers already sent by (output started at /home/herozero/html/2/wp-content/themes/HZV02IS01/functions.php:1) in /home/herozero/html/2/wp-includes/pluggable.php on line 694

How can i solve this? And why is wp doing this? It’s not like you need the theme when accessing your admin right?

Help!

Thanks in advance…