When you are writing a host-side add-on for PHost, you should definitely have a look at the PHost Development Kit (PDK). The PDK encapsulates all the logic of accessing host data files, so you can start concentrating on your add-on, not on boring things like file I/O. The PDK's goal is to be compatible with Tim's host, but probably does not yet deliver 100% compatibility. Another goal is to provide full access to all PHost specialties, but that goal has not been reached yet as well.
In any case, you should have a look at the File Format List. This list contains a complete description of almost all Planets-related file formats, for HOST and PHost. It was compiled by Stefan Reuther (now a PHost author) mostly by reverse-engineering, so it generally provides you with a more critical view than official PHost docs ;-)
