sybase_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 sybase_connect() was called, and use it.
如果成功则返回 TRUE，失败则返回 FALSE。
Every subsequent call to sybase_query() will be made on the active database.