Anthony Wlodarski makes an important point about what one of your first steps should be when considering a framework for your application - be sure to read the source first. In his case, it was the Zend Framework.
One thing I learned early on with ZF was that the curators and associates in the ZF ecosystem always fall back to the root of "read the code/api/documentation". With good reason too! It is not the volunteers simply shrugging you off but it is for your own good.
He includes an example of the Zend_Validate_File_ImageSize component. By reading the manual you get an idea of the input and output values, but you don't get as much of a sense of how it works unless you take some time to glance through the code.