msql_select_db() sets the current active database on the server that's associated with the specified link_identifier. If no link identifier is specified, the last opened link is assumed. If no link is open, the function will try to establish a link as if msql_connect() was called, and use it.
如果成功则返回 TRUE，失败则返回 FALSE。
Subsequent calls to msql_query() will be made on the active database.
For downward compatibility, the alias named msql_selectdb() may be used. This, however, is deprecated and not recommended.
See also msql_connect(), msql_pconnect(), and msql_query().