PHP et checkbox

<?php
  $form_valid = true;
  if($_SERVER['REQUEST_METHOD'] == 'POST') {
    if(isset($_POST['nom']) && ($_POST['firstname']) && ($_POST['birthday']) && ($_POST['email']) && ($_POST['engagement']) && ($_POST['duree'])) {
      if(!empty($_POST['nom']) && ($_POST['firstname']) && ($_POST['birthday']) && ($_POST['email']) && ($_POST['engagement']) && ($_POST['duree'])) {
        if ($_POST['checkbox'] == ) {
          # code...
        }
        $bdd = new PDO('mysql:host=xxxxx;dbname=yyyyyyl;charset=utf8', 'yyyyyy', 'zzzzz');

        $req = $bdd->prepare('INSERT INTO `posticpaul`.`jeunes` (`firstname`, `lastname`, `birthday`, `mail`, `type`, `duration`, `qualities`) VALUES (:firstname, :lastname, :birthday, :mail, :type, :duration, :qualities)');
        $req->execute(array(
         'firstname' => $_POST['firstname'],
         'lastname' => $_POST['nom'],
          'birthday' => $_POST['birthday'],
         'mail' => $_POST['email'],
         'type' => $_POST['engagement'],
         'duration' => $_POST['duree'],
         'qualities' => "",//Ajout des qualités
         ));
      } else {
        $form_valid = false;
      }
    } else {
      $form_valid = false;
    }
  }
?>

    
      <form class="formulaire" method="POST" action="">
         <div class="coche">
          <p class="i_am">
            Je suis...
          </p>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Autonome <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Créatif <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Honnête <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Passionné <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Réfléchi <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Fiable <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Patient <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Sociable <br>
          <input type="checkbox" class="valid" name="checkbox" onclick="verif(this)"> Optimiste <br>
          <em id="nouveau"></em>
          <input type="text" id="rajout" name="rajout" value="Ajouter une qualité" onclick="rajout.value=''">
          <button type="button" name="rajout1" onclick="ajout()">✓</button><br>
          <p class="quatre_choix">
            Faire quatre choix au maximum
          </p>
          <button type="submit" class="valider_jeune">Valider vos réponses</button>
        </div>
      </form>

Ligne 11 code php xxxxxx, yyyyyy, zzzzz  = adresse, identifiant et mot de passe  supprimés Modération CCM