使用PDO预处理语句和遍历接口时，如何指定fetch_style？

My pdo connection is set up to return standard objects, and I can use the traversal interface to iterate over the results as follows:

$stmt=$this->pdo->prepare($sql);
$stmt->execute($arr);
foreach($stmt as $rs) {
    //$rs is a stdObject since the pdo connection is set up to return stdObjects
}

Per http://php.net/manual/en/pdo.query.php, I can specify the fetch_style and similarily itterate over the results as follows:

$stmt = $pdo->query($sql, PDO::FETCH_ASSOC);
foreach($stmt as $rs) {
    //$rs is an array even though pdo connection is set up to return stdObject since I included `PDO::FETCH_ASSOC`
}

What I want, however, is to use a prepared statement when my pdo connection is set up to return standard objects, and use the traversal interface to iterate over the results and return arrays:

$stmt=$this->pdo->prepare($sql);
$stmt->execute($arr);
foreach($stmt as $rs) {
    // pdo connection is set up to return stdObject.  What can be done to make $rs an array?
}

How can this be done?

Note that typecasting to an array is not an acceptable answer as not using the traversal interface in the first place would be a better approach.

However, stating that it is not possible (if true) is an acceptable answer.

Your first stop should always be the PHP manual ~ PDOStatement::setFetchMode

$stmt=$this->pdo->prepare($sql);
$stmt->execute($arr);
$stmt->setFetchMode(PDO::FETCH_ASSOC);

foreach($stmt as $rs) {
    // $rs is an array
}

This will set the fetch-mode only for the particular PDOStatement instance and will not have any affect on future queries.