well, there's a couple things here...
First, when you add a field to an existing table, eben though you set what the default setting should be, it does not set it for existing entries, so you will have to update them:
update user set tos='n';
this will update all records
second:
I don't see your connection info or select statement, so im gonna ask you to make sure you're assigning a value to $user before you reference to it:
// example
$query = mysql_query("select tos from user where id='0'");
$row=mysql_fetch_array($query);
$user = $row[tos];