I hope no one looks at this as advice... I was in this position for a long time, and looking back, it's a completely unsustainable (and, in my opinion, irresponsible) method of development.
Problems with PHP itself aside, rolling your own from scratch only ever works out well for projects that start small and stay small. Even if the code is sane and readable, a developer starting with a site like this will need to understand either all of the patterns that are being used or to read and understand all of the code before being able to confidently and effectively work with it.
Contrast this to a framework, where similarly-sane code will yield something anyone with experience in the framework can almost-immediately begin working with.
Both have an up-front cost; however, the first incurs that cost every time any new developer starts working with any project done in such a way. The second is incurred once, period, making the startup time on any project on the same framework dramatically lower.
Frameworks are by no means a silver bullet, but their utility should not be underestimated, and the very nasty maintenance cost effects of the parent's post should be similarly understood.