¿Cómo se generan resultados combinados de dos tablas MySQL usando PDO?

Ahora la lógica de la aplicación será

<?php
  $pin = 123;
  $db = new PDO('mysql:host=localhost;dbname=test', 'username', 'password');
  $stmt = $db->prepare(
    'SELECT 
        conf.conference_sid,
        date_created, 
        timediff(date_completed, date_created) AS duration,
        conf.RecordURL, 
        conf.PIN,
        GROUP_CONCAT(pid SEPARATOR ",") AS pid,
        GROUP_CONCAT(call_sid SEPARATOR ",") AS call_sid,
        GROUP_CONCAT(call_from SEPARATOR ",") AS call_from,
        GROUP_CONCAT(name_recording SEPARATOR ",") AS name_recording
    FROM 
      conference conf
    LEFT OUTER JOIN 
      participants p ON p.conference_sid = conf.conference_sid
    WHERE 
      conf.PIN = :pin
    GROUP BY conf.conference_sid');
  $stmt->bindParam(':pin', $pin);
?>
<table border="1">
<thead>
  <th>Date</th>
  <th>Duration</th>
  <th>Participants</th>
  <th>Recording</th>
</thead>

<tbody>
<?php
  $stmt->execute();
  while ($row = $stmt->fetch()) {
?>
    <tr>
      <td><?php echo $row['date_created']; ?></td>
      <td><?php echo $row['duration']; ?></td>
      <td>
        <table border="1">
          <thead>
            <th>call_sid</th>
            <th>call_from</th>
            <th>name_recording</th>
          </thead>
          <tbody>

<?php
    $length = count(explode(',', $row['pid']));
    $call_sid = explode(',', $row['call_sid']);
    $call_from = explode(',', $row['call_from']);
    $name_recording = explode(',', $row['name_recording']);
    for ($i=0; $i < $length; $i++) { 
      ?>
        <tr>
          <td> <?php echo $call_sid[$i]; ?> </td>
          <td> <?php echo $call_from[$i]; ?></td>
          <td> <?php echo $name_recording[$i]; ?> </td>
        <tr>
<?php
    }
?>
        </tbody>
        </table>
      </td>
      <td>
        <a href="<?php echo $row['RecordURL']; ?>">
        Call recording</a>
      </td>
    </tr>
<?php
  }
?>
</tbody>