$comments = $wpdb->get_results("SELECT * FROM ".wp_table('comments')." WHERE comment_post_ID = '$wp_post_id' AND comment_approved = '1' AND (comment_content like '<trackback />%' OR comment_content like '<pingkback />%') ORDER BY comment_date");
を
$comments = $wpdb->get_results("SELECT * FROM ".wp_table('comments')." WHERE comment_post_ID = '$wp_post_id' AND comment_approved = '1' ORDER BY comment_date");