このコードですか
/* already set up a query with one column in result */
$row['fk_user_pks'] = $q->fetchAll(PDO::FETCH_COLUMN);
に相当:
/* already set up a query with one column in result row */
$result = $q->fetchAll(PDO::FETCH_NUM);
$row['fk_user_pks'] = array();
foreach ($result as $r) {
$row['fk_user_pks'][] = $r[0];
}