there are many database operation on a controller. every db operation operate by clone ($_db = clone $this -> db). the page show some notice:

A PHP Error was encountered

Severity: Warning

Message: mysql_errno(): 31 is not a valid MySQL-Link resource

Filename: mysql/mysql_driver.php

Line Number: 470
A PHP Error was encountered

Severity: Warning

Message: mysql_error(): 31 is not a valid MySQL-Link resource

Filename: mysql/mysql_driver.php

Line Number: 457

A Database Error Occurred

Error Number:

SELECT * FROM ....

when i delete some db operation , the page shows correct.
is there anyone meets this problem?