The day I have been dreading has arrived. I never felt OOP or good software design was important(I knew they were important, but I thought I could manage without them.). However having read otherwise almost everywhere on the interwebs, I started dreading the day when my client would ask me for new features in an existing app. The day has come and the pain is unbearable!

I have never coded my PHP websites "properly"(PHP is my primary language and the bulk of my work. I am learning Python (using web2py)) I take care that the website doesn't fall apart in a daily use scenario. I code pages like I was creating a list of static html files with bits of "magic code" in each of them(this bugs me a lot). How do I make the whole app more or less a single object? For eg. How do I design the object model for an invoicing app?

I use a lot of functions for doing any particular thing in the same fashion throughout the app(for eg. validation, generating ids, calculating taxes etc.).

I know the basics of OOP in general. Can anyone point me to source code samples of functional apps written in php? Or can someone provide pointers so I can recode my existing apps in a more modular way.