Pdo: support setFetchMode()

[staabm]

<?php
$stmt = $dbh->query('SELECT name, colour, calories FROM fruit');
$stmt->setFetchMode(PDO::FETCH_NUM);
foreach ($stmt as $row) {
    print $row[0] . "\t" . $row[1] . "\t" . $row[2] . "\n";
}

https://www.php.net/manual/en/pdostatement.setfetchmode.php

similar to #253