Simple sums

[MitchEvans]

How do I do simple sums using php? I want to add certain data in my database together where the id's are the same. For example:
//id// //total//
01 5
01 4

So here i would add 5+4 together where id = "01".

[Crowly]

Heres one way

$var1= 4;
$var2= 5;
$sql = sprintf("UPDATE some_table SET field = %d + %d WHERE id = 1",$var1,$var2);
mysql_query($sql);

In case i missunderstood

$query="SELECT sum(some_field) FROM table WHERE id=1";
$result=mysql_query($query);
$sum=mysql_fetch_row($result);

$query=sprintf("UPDATE table SET field = %d WHERE id = 1",$sum[0]);
mysql_query($query);

[laserlight]

I believe it would look something like:

SELECT SUM(total) FROM tablename WHERE id=1;

Be warned that I am really rusty on my SQL, and hence prone to silly mistakes.