Script PHP para calcular totais cumulativos para contas

SELECT
    CusName,
    CusID,
    Quantity,
    (SELECT SUM(t2.Quantity) FROM yourTable t2
     WHERE t2.CusID = t1.CusID AND t2.Date <= t1.Date) QuantityTotal,
    Date
FROM yourTable t1
ORDER BY
    CusID,
    Date;