currently I am developing a reminder system for unpaid bills, completely programmed in PHP (OOP), running on our intranet, with a MySQL backend
it is a fairly complex system, generating the reminders in PDF-format, and distributing them by email, fax or snail-mail (printed PDF), depending on what addressing-information we have on file of the client
the system has an administration-panel from where the data of the bookkeeping-system is being read, the reminder-batch is being started etc.
administration has many features such as blocking certain clients, generating so called 'blacklists' of clients which do not pay after X-times, resending and reprinting the reminders, change client-details and many many more
this is the biggest web-based-application I have ever programmed š
since the system is very complex, I wrote detailed technical specifications, before I even started to code a single line
writing the specs I ran into some problems, such as : how to print a PDF from PHP on a remote network printer, which works fine now š
so if you want to develop such a 'big pain in the *ss', I suggest you write the specs first and solve implementation-issues before you code a single line, and to prevent headaches, use PHP-OOP