Help! pdo:prepared()->execute() didn't return rows.

Dear ALL, My PHP is running under ubuntu 11.10 in VituralBox. The version returned from phpinfo() is: PHP Version 5.3.6-13ubuntu3.1 PDO supported for mysql. pdo_mysql Client API version is: 5.1.58. I wrote a stored procedure to insert a row into a table, then select the inserted row at the end of the procedure. PDO::query() works fine. I change it to PDO:prepared() and PDOStatement->execute() for better performance. but the PDOStatement->fetch() return false after true was got at PDOStatement->execute(). Could somebody help me, please!!! Thank you. Best Regards, Antony