sybase_close() closes the link to a Sybase database that's associated with the specified link link_identifier. If the link identifier isn't specified, the last opened link is assumed.
如果成功则返回 TRUE，失败则返回 FALSE。
Note that this isn't usually necessary, as non-persistent open links are automatically closed at the end of the script's execution.
sybase_close() will not close persistent links generated by sybase_pconnect().