php script modify for error

I have a web base software running over my localhost. I am haveing problem with create invoice. Problem are as follow: Problem 1. User 1: creating an invoice with product X, Y, Z and User 2 : creating an invoice A,B,C. Problem is when both user saving this invoice, User 1 or user 2 products merge with user 2 or user 1 products and the invoice is like that, X,Y,Z,A,B,C. I need individuals user should print individuals, that products should merge with another user. Problem 2: The invoice is saving in database in two difference table. one is invoice and another is invoice details. Invoice saving in invoice details table properly but some time taking wrong saving in invoice table. Details will be provide through team viewer or skype.