int mssql_select_db(string database_name, int link_identifier);
Returns: true on success, false on error
mssql_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 mssql_connect() was called, and use it.
Every subsequent call to mssql_query() will be made on the active database.