Unable to set custom PDO instance

$pdo->setAttribute(PDO::ATTR_STATEMENT_CLASS, array('Doctrine\DBAL\Driver\PDOStatement', array()));

<?php
use Doctrine\ORM\Tools\Setup;
use Doctrine\ORM\EntityManager;

require __DIR__ . '/Doctrine/ORM/Tools/Setup.php';
require __DIR__ . '/Entities/Foo.php';

Doctrine\ORM\Tools\Setup::registerAutoloadPEAR();

$paths = array("Entities");
$isDevMode = false;

$pdoInstance = new PDO('mysql:host=localhost;dbname=mydb', 'root', 'mypass');

// the connection configuration
$dbParams = array(
    'driver'   => 'pdo_mysql',
    'pdo'      => $pdoInstance
);

$config = Setup::createConfiguration($isDevMode);
$driverImpl = $config->newDefaultAnnotationDriver('/Entities');
$config->setMetadataDriverImpl($driverImpl);

$em = EntityManager::create($dbParams, $config);

$query = $em->createQuery('SELECT e FROM Foo e');
$query->useResultCache(true);
$query->setResultCacheLifetime(3600);
$query->setResultCacheDriver($em->getConfiguration()->getQueryCacheImpl());

// Below line is important. It force query to use SQLWalker which use ResultCacheStatement.
$query->setHint($query::HINT_CUSTOM_TREE_WALKERS, array('\Doctrine\ORM\Tools\Pagination\CountWalker'));

$query->getResult();

 
[...]

$pdoInstance = new PDO('mysql:host=localhost;dbname=somedb', 'root', 'mypassword');

[...]

$connection['driver'] = 'pdo_mysql';
$connection['pdo']    = $pdoInstance; 

[...]

$config = Setup::createConfiguration();

[ Setup annotation driver, cache setup etc. ]

$entityManager = EntityManager::create($connection, $config,  $evm);

$connection['driver']   = 'pdo_mysql';
$connection['user']     = 'root';
$connection['password'] = 'mypassword';