All I can see is that the code listed is breaking at "&amp"
Here is what I have for it and I get /users.php?action=edit&user=8
<?php if ($action != 'delete') { ?>
<a href="<?php echo zen_href_link(FILENAME_USERS, 'action=edit&amp;user=' . $userDetails['id']) ?>"><?php echo zen_image_button('button_edit.gif', IMAGE_EDIT) ?></a>