I am trying to generate a bank NACHA (national automated clearing house association) file with my customers payment details to submit to my bank on a regular basis. Our site
Most businesses that need to create these files are .net houses. You might have better luck finding something in C# and reverse-engineering it.
The National ACH Association "NACHA" publishes the ACH rules on an annual basis. The current version, "2012 NACHA OPERATING RULES & GUIDELINES", contains the ACH rules, along with ACH file formats (i.e. Reference Guide) for creating your own tools to build ACH files.
http://www.achrulesonline.org/
I also needed such a thing... didn't find one so I made one.
https://github.com/snsparrish/NachaLibPHP