or mysql_fetch_row or mysql_fetch_array.
Using mysql_fetch_row, you can do
$result = mysql_query($sql);
$row = mysql_fetch_row($result);
$yourNumber = $row[0];
If you use mysql_fetch_array, you'll need to give the calculation an alias, like
$sql = "SELECT sum(rat_0 + rat_1 + rat_2 + rat_3 + rat_4 + rat_5) AS total
FROM table";
$result = mysql_query($sql);
while($row=mysql_fetch_array($result)) {
$yourNumber = $row['total'];
}