Classic PHP vs Framework (Kohana,Cake,CI) vs PEAR?

If you intend on maintaining complicated code for a long time, a framework is essential. You can build one up yourself or you can use an existing one. Existing ones are well developed and used by lots of developers so you can ask questions on sites like Stackoverflow.com if you get stuck.

As for which of those frameworks is better.. I think it all comes down to choice. In the end you will have to pick one and get started. Don't spend too long on this decision (Think a lot about it but don't let it stop you from starting!)