I prefer my own library of functions
rather than having a framework
You refer to a 'library of functions' and 'a' framework' as distinct concepts (good!) so why not try and up the ante a bit and turn that library of functions into a framework? Mind you this is a great way to learn but not the best way to have marketable skills ;)
It's also a great way to learn wrong so before you start you have to do some research.
The "must have" features
There are certain features almost all frameworks have, and there's a reason for that. Research them; find out why they are present and what their variations are; choose which implementations you'd prefer to include in your framework.
- Object Oriented (this is key to why you'd write a framework as opposed to just using your library of functions)
- Use of Patterns (check out and start asking yourself why so many frameworks use Application Controller as opposed to Front Controller... and are there some platforms which use Page Controllers?
- Basic MVC structure
- Clean URLs
Features of non-PHP platforms you want to understand and emulate
- If you like Ruby perhaps you'd want to a means by which to include modules in classes?
- If you like Asp.NET maybe you'd want to try creating Server Controls in a PHP environment?
- Upset that forgiveness is the default with HTML? Why not validate output server side
I have no idea what interests you so I'll stop there.
Publish it!
This part is important, because Nathan is right: your work has to be seen, challenged, critiqued and corrected by other people. Don't leave that lamp under a bushel - it needs to be seen; put it on Google code or sourceforge or something, and if people are going to bother looking at it seriously you need to take it seriously; you need to maintain unit tests and regression tests; you need to use version control; you need to comment it well, with a big /* header comment */
at the top of each file and useful doc-generating comments for each class and member thereof.
Summary
If you do all this, you can learn (at a high level) about other platforms. You can definitely "ding 40" as a PHP developer. You can learn about unit testing; about regression testing. You can learn about documenting effectively.
Guaranteed your framework will come out "idiosyncratic and probably sucky", but you can learn a lot from writing it. Just keep in mind that the framework isn't a product; it's a journey... of sorts.