Invoice Number

[barb]

i am trying to add an invoice number to my form from a file.

I want to open the file, get the value, add it my form, increment the number
and write it back to my form.

Then submit a copy of the form to the person submitting it and another email
address and I want the invoice number to appear in the invoice number field.

Can anyone help me?
I can open and read the text file and write back to it but I can not display
the number or increment it.