带有2个下拉列表的外键

I have a problem concerning my drop-down list in PHP. I have in fact 2 tables.

The first is the table feedbacks with several fields (id_feedback, lesson_feedback, description_feedback, fk_candidat)

My second table is candidats with 3 fields (id_candidat, name_candidat, firstname_candidat)

When I want to add a recordery from my form feedbacks. I must to get the fields below lesson_feedback, description_feedback, name_candidat (with a drop-down-list), firstname_candidat (with a drop-down-list).

My problem is I don't recover the fk_candidat for the firstname_candidat. I recover the fk_candidat for the name_candidat only. I need of the name and of the firstname.

Here is an idea of the code

if(isset($_POST['lesson_feedback']) AND isset($_POST['description_feedback']) AND isset($_POST['fk_candidat'])){

   $lesson_feedback = htmlspecialchars($_POST['lesson_feedback']);
   $description_feedback = htmlspecialchars($_POST['description_feedback']);
   $fk_candidat = ($_POST['fk_candidat']);

   $req_add = $db->prepare("INSERT INTO feedbacks (lesson_feedback,description_feedback, fk_candidat) VALUES (?,?,?)");

   $req_add->execute(array($lesson_feedback, $description_feedback, $fk_candidat));

}

$sql = $db->query("SELECT * FROM candidats");

In HTML

<form action="add_feedbacks.php" method="POST">
<table>
   <tr><td>Lesson:</td><td><input type="text" name="lesson_feedback'"/></td></tr>
   <tr><td>Description:</td><td><input type="text" name="description_feedback'"/ </td></tr>
   <tr><td>Name candidat:</td><td><select name="fk_candidat" style="width:148px">
   <?php

   while($row = $sql->fetch()) { 
   ?>

   <option value="<?= $row['id_candidat']; ?>"><?= $row['name_candidat'];?>    </option>

   <?php
   }
   ?>
   </select>

   <tr><td>Firstname candidat:</td><td><select name="fk_candidat" style="width:148px">
   <?php
   while($row = $sql->fetch()) { 
   ?>
   <option value="<?= $row['id_candidat']; ?>"<?= $row['firstname_candidat'];?></option>
  <?php
  }
  ?>
  </select>

  <tr><td colspan="2"><input class="button" type="submit" value="Ajouter"/> 
  </td></tr>
</table>

</form>

I think my problem is here ?

<tr><td>Firstname candidat:</td><td><select name="fk_candidat" style="width:148px">
       <?php
       while($row = $sql->fetch()) { 
       ?>
       <option value="<?= $row['id_candidat']; ?>"<?= $row['firstname_candidat'];?></option>
      <?php
      }
      ?>
      </select>