Para coger el id siguiente:
$sql = sprintf("SELECT id FROM `wpd_noticies` WHERE id > (SELECT id FROM `wpd_noticies` WHERE id LIKE %s ORDER BY id DESC)", $id);
Para coger el id anterior:
$sql = sprintf("SELECT id FROM `wpd_noticies` WHERE id < (SELECT id FROM `wpd_noticies` WHERE id LIKE %s ORDER BY id DESC)", $id);
