Those unfamiliar with socket programming can find a lot of useful material in the appropriate Unix man pages, and there is a great deal of tutorial information on socket programming in C on the web, much of which can be applied, with slight modifications, to socket programming in PHP. The UNIX Socket FAQ might be a good start.

Adding Hyperwave support to PHP should fill in the gap of a missing programming language for interface customisation. It implements all the messages as defined by the HG CSP but also provides more powerful commands to e.g. retrieve complete documents.

Assertions should be used as a debugging feature only. You may use them for sanity checks that test for conditions that should always be TRUE and that indicate some programming errors if not or to check for the presence of certain features like extension functions or certain system limits and features.