What does the --server command line option do? It is not documented, yet is used in the php wrapper. Also the forum doesnt allow a search for the term "--server"

It changes the format of log messages to make them easier to parse by wrapper software. It's not the most obvious name for an option, as it was originally intended to perform other server-related tasks as well.

Does the php wrapper depend on this option I wonder?

Yes, so that it can parse the log messages and return them in the msgs array.