Description:
------------
It mounts in the class. 

       session_set_save_handler(
           array($this, '_open'),
           array($this, '_close'),
           array($this, '_read'),
           array($this, '_write'),
           array($this, '_destroy'),
           array($this, '_clean')
       );

...

public function _write( $strID, &$objData ) {

   // some process.
   throw new Exception;
}

Result:
Exception thrown without a stack frame in ....

Why?
The exception is normally generated in other methods. 
ex. _read,_destroy,_clear... 

Reproduce code:
---------------
    public function _write( $strID, &$objData ) {

        try {
            $strSQL = "REPLACE INTOd sessions hogeVALUES ( ?, ?, ?  )";

            $putData = $this->objResource->prepare( $strSQL );
            $putData->bindParam( 1, $strID );
            $putData->bindParam( 2, time() );
            $putData->bindParam( 3, $objData );
            $putData->execute();
            return true;
        } catch( PDOException $e) {
            $strError = sprintf( "?Z?b?V?????f?[?^?̐ݒ??Ɏ??s???܂????B<br><br>%s" ,$e->getMessage() );
            throw new Exception( $strError );
        }
   }

Expected result:
----------------
Fatal error: Exception thrown without a stack frame in Unknown on line 0