The socket extension implements a low-level interface to the socket communication functions based on the popular BSD sockets, providing the possibility to act as a socket server as well as a client.
For a more generic client-side socket interface, see stream_socket_client(), stream_socket_server(), fsockopen(), and pfsockopen().
When using these functions, it is important to remember that while many of them have identical names to their C counterparts, they often have different declarations. Please be sure to read the descriptions to avoid confusion.
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.