When Insert .. but what when Update ?

[prokhaled]

In this code:
$Sql = "INSERT INTO Subject (ReplyNo) VALUES ('0')";
$Result = mysql_query($Sql);
Echo mysql_insert_id();

I use ( Echo mysql_insert_id(); ) to echo the the current ID that Inserted

but what can I use to know the current ID that updated ??

I try that
$Sql = "UPDATE Main SET tt='$tt' WHERE MainID=$MainID";
$Result = mysql_query($Sql);
Echo mysql_update_id();

but give me error that (there is not any function like that (mysql_update_id()