如何在事务中获取MySQL查询的结果，然后获取其下一部分的结果？

I'm rying to fech the results in the middle of a transaction, and then fech the results of the second part of the transaction. How may I do that?

I'm using PHP PDO and MySQL.

      $conectar1 = new PDO('mysql:host='.HOST.'; dbname='.DATABASE.'; charset=utf8', USER, PASS); 
      $mostrarExamen = $conectar1->beginTransaction();
      $mostrarExamen = $conectar1->prepare("
            SELECT examenID, userID
            FROM examenes
            WHERE userID = ?
        ");
      $mostrarExamen->bindParam(1, $userID);
      $mostrarExamen->execute();

      $examen = $mostrarExamen->fetch(PDO::FETCH_ASSOC);
      $examenID = $examen['examenID']; 

      $mostrarExamen = $conectar1->prepare("
            SELECT cursosUsuarios.examenes AS respuestasdelusuario
            FROM cursosUsuarios LEFT JOIN cursos
            ON cursosUsuarios.cursoID = cursos.cursoID
            AND cursosUsuarios.cursoID = ?
      ");     
      $mostrarExamen->bindParam(1, $cursoID);
      $mostrarExamen->execute();

//I'VE TRIED FETCHING THE RESULTS AGAIN
      $resultado = $mostrarExamen->fetch(PDO::FETCH_ASSOC);

//THIS IS NULL
echo $resultado['respuestasdelusuario'];

      $mostrarExamen = $conectar1->commit();