Hi hi, I'm looking for a easy to learn php library to use for my coming web app project. I've recently finished a web app with fully handwritten raw php code and it's absolutely hard to be done again for another project.
even though I have the recent project code snippets to be used again, but due to their non-structural arrangement (not object oriented), i have no passion to use 'em again.
I have no experience with common frameworks like ZF, CakePhp, CodeIgniter, so I think to get my hands on a multipurpose OO library for my web app and the framework learning will be the next step! any suggestion?

UPDATE:
Many thanks guys, I have not enough time to get through the depth of every lib or framework you have kindly introduced. Since I'm going one step further I'm going to use ZF as famous framework which could provide me more job opportunities perhaps. thankssss :)