A basic PHP/MySQL/HTML Billing APP. Meant for developers learning.
Here is PHP/MySQL/HTML code that is a sample Entry Level Billing APP. It is Free, and mostly for learning purposes. You can install on a Windows or linux server, and look at the PHP code files on your PC.
...The appliance is built using scripts so no hidden "monkey business" and you can choose a "compatible base distro" based on your needs: familiarity; performance; security; usability; etc. Please see https://sourceforge.net/p/formavid/code/ci/master/tree/INSTALL.txt for details. The appliance is specially designed to run on the Google Cloud Platform "Free Tier" https://formavid.org/gce and has it's own custom install script https://sourceforge.net/projects/formavid/files/gcloud-gce-deployment/download with instructions https://sourceforge.net/projects/formavid/files/ provided.