In MySQL, this does return the highest value from the id column:
SELECT MAX(id) FROM tablename;
However, this does not put that id into $n:
$n = mysql_query("SELECT max(id) FROM tablename");
To get the value, you need to do this:
$result = mysql_query("SELECT max(id) FROM tablename");
if (!$result) {
die('Could not query:' . mysql_error());
}
$id = mysql_result($result, 0, 'id');
If you want to get the last insert ID from A, and insert it into B, you can do it with one command:
INSERT INTO B (col) SELECT MAX(id) FROM A;