PHP Foundation Classes
Quality Packages must be in a stable state.
Generality Packages should not be excessively specific to any particular type of environment.
Interoperability Packages should work well with other packages and in different environments, and they should have a standardized API.
Compatibility Packages must be designed to be backward compatible when new features are added.
At the present time, only the PEAR installer is shipped with PHP. However, at a later date, certain classes may be included as standard. The PFC would be those classes.