$status = $_POST['status'];
if(!empty($_POST['status'])){
foreach($_POST['user_id'] as $id){
mysql_query("UPDATE $tb_user SET status = '$status' WHERE user_id = '$id'") or die (mysql_error());
}
$status = $_POST['status'];
if(!empty($_POST['status'])){ // เพราะตรงนี้ครับ
foreach($_POST['user_id'] as $id){
mysql_query("UPDATE $tb_user SET status = '$status' WHERE user_id = '$id'") or die (mysql_error());
}