query_read_slave sets the database object with a cursor, it does not return the actual values.
also when not doing a select you should use query_write. (I think...dont quote me on this one)
PHP Code:
// sql coins test
$db->query_write("update user set coins=5 where userid=1");
$coins_query = $db->query_read_slave("select coins from user where userid=1");
while ($coins = $db->fetch_array($coins_query))
echo $coins['coins'];