无法使用PDO在PHP中运行嵌套查询（从第一个查询的每个输出中获取新的结果集）

I'm trying to upgrade from PHP v5.4 to php v7.0, where I ran into this issue of being unable to run the inner nested query. I'm was using pdo_sqlsrv for my SQL server but since the PDO drivers for it aren't yet available for PHP v7.0, I switched to pdo_odbc. Is being able to run nested queries specific to sqlsrv? I've added some test code to make my situation clear. DBTEST class for the PDO instance:

class DBTEST{
    private static $instance = null;
    public static function getInstance(){
        $userid = "testUser";
        $pwd = "love-da-poe-poes";
        self::$instance = new PDO("odbc:testdb", $userid, $pwd);
        return self::$instance;
    }
}

Nested Queries:

$parentQuery = "Select pid from Tree";
$parentResult = DBTEST::getInstance()->prepare($parentQuery);
$parentResult->execute();

$childQuery = "Select cid, cname from Tree where pid = ?";
$childResult = DBTEST::getInstance()->prepare($childQuery);

while($parentObject = $parentResult->fetchObject()){
    $childResult->bindParam(1, parentResult->pid, PARAM_INT);
    $childResult->execute();
    while($childObject = $childResult->fetchObject()){
        //do something with results
    }
}