This function returns a socket error code.
If a socket resource is passed to this function, the last error which occurred on this particular socket is returned. If the socket resource is omitted, the error code of the last failed socket function is returned. The latter is in particular helpful for functions like socket_create() which don't return a socket on failure and socket_select() which can fail for reasons not directly tied to a particular socket. The error code is suitable to be fed to socket_strerror() which returns a string describing the given error code.
Note: socket_last_error() does not clear the error code, use socket_clear_error() for this purpose.