循环执行时PHP存储过程错误

Question

I have the PHP script :

<?php       
    $count=1;
    $result = $db_connection->prepare("SELECT tid, title FROM message");
    $result->execute();

    while($row = $result->fetch())
    {
        $co_count = 0;
        $cosql = $db_connection->prepare("CALL sp_getresp(?);");
        $cosql->execute(array($row['tid']));

        while($row3 = $cosql->fetch())
        {
            // GET USERS INFO
            $couser = $db_connection->prepare("CALL sp_getuserinfo(?);");
            $couser->execute(array($row3['couid']));
            $row4 = $couser->fetch();

            var_dump($row4);
            echo '<BR>'.$row['tid'].$row4['email'].'<BR>';
            $count++;

        }
    }
?>

In the above code, I try to run the stored procedure. It is working fine except, in the first loop, it doesnt give any data even the data is present. I tried many tests, but $row4['email'] always shows empty in the first loop time, after first loop, it runs fine. I also check directly calling CALL sp_getuserinfo(?); in the mysql where it works fine but not here. I dont understand what is the problem here ?