Fatal error: Using $this when not in object context in C...

Bonjour,

Je suis en train de m'initier à la POO, j'ai construit la connexion à ma BDD, ensuite une class qui sert à enregistrer les membres et j'ai cette erreur je crois comprendre pourquoi mais je n'arrive pas à le résoudre, peut-être que $this n'est pas définit dans un objet?
Pouvez-vous me donner quelques explication qui pourrais m'être utile s'ils vous plait.
En vous remerciant.
L'erreur à la ligne 5

 Fatal error: Using $this when not in object context in C

ma class Inscription

class Inscription {
 
 public function enregistrer() {

  $req = $this->db->prepare('SELECT COUNT(pseudo) AS nbre_pseudo FROM tb_adherent WHERE pseudo = ?');
  $req->execute(array($_POST['pseudo']));
  $result = $req->fetch();

  if(empty($_POST['nom']) OR empty($_POST['prenom']) OR empty($_POST['pseudo']) OR empty($_POST['password']) OR empty($_POST['debut'])) {
   header('Location: index.php?error=empty');
   
  } else if($result['nbre_pseudo'] > 0) {
    header('Location: index.php?error=pseudo_exist');
    
  } else {
   $req->closeCursor();
   
   $nom   = Security::clean($_POST['nom']);
   $prenom  = Security::clean($_POST['prenom']);
   $email   = Security::clean($_POST['email']);
   $naissance  = Security::clean($_POST['naissance_3']) ? trim($_POST['naissance_3']) . '-' .  trim($_POST['naissance_2']) . '-' .  trim($_POST['naissance_1']) : '';
   $fixe   = Security::clean($_POST['fixe']);
   $mobile   = Security::clean($_POST['mobile']);
   $adresse  = Security::clean($_POST['adresse']);
   $lieu_dit  = Security::clean($_POST['lieu_dit']);
   $code_postal = Security::clean($_POST['code_postal']);
   $ville   = Security::clean($_POST['ville']);
   $pseudo  = Security::clean($_POST['pseudo']);
   $password  = Security::clean(Security::hash($_POST['password']));
   $actif   = $_POST['actif'];
   $debut   = Security::clean($_POST['debut']);
   $fin   = Security::clean($_POST['fin']);
   
   $req = $this->db->prepare('INSERT INTO tb_adherent (nom, prenom, email, naissance, fixe, mobile, adresse, lieu_dit, code_postal, ville, pseudo, password, actif, debut, fin) VALUES (:nom, :prenom, :email, :naissance, :fixe, :mobile, :adresse, :lieu_dit, :code_postal, :ville, :pseudo, :password, :actif, :debut, :fin)');
   $req->execute(array(
    'nom'    => $nom,
    'prenom'   => $prenom,
    'email'   => $email,
    'naissance'  => $naissance,
    'fixe'    => $fixe,
    'mobile'   => $mobile,
    'adresse'   => $adresse,
    'lieu_dit'   => $lieu_dit,
    'code_postal'  => $code_postal,
    'ville'   => $ville,
    'pseudo'   => $pseudo,
    'password'   => $password,
    'actif'   => $actif,
    'debut'   => $debut,
    'fin'    => $fin
    //$resultat -> bindParam(':pseudo', $identifiant);
   ));
   $req->closeCursor();
   
   header('Location: index.php?info=enregistrer');
  }
 }
}

ma class Db et mon constructeur

class Db {
 
 private $db;
 
 public function __construct() {
  $this->db = new Connection();
  $this->db = $this->db->connect();
 }
}

mon formulaire
inscription.php

<?php
if(isset($_POST['send'])) {
 extract($_POST);
 echo Inscription::enregistrer();
 
 if(isset($_GET['error']) AND $_GET['error'] = 'empty') {
  echo 'Veuillez remplir correctement le formulaire';
 }
}
?>
<div id="content">
 
 <form id="formulaire" action="" method="post">
        <fieldset>
            <legend>Ajouter un membre</legend>

  <table cellpadding="0" cellspacing="3">

   <tr>
    <th colspan="6">Coordonné :</th>
   </tr>
   <tr>
    <td width="20%">Nom :</td>
    <td width="30%"><input name="nom" class="content_input" size="30" type="text" value=""/></td>
    <td width="20%">Prénom :</td>
    <td width="30%"><input name="prenom" class="content_input" size="30" type="text" value=""/></td>
   </tr>
   <tr>
    <td width="20%">Adresse :</td>
    <td width="30%"><input name="adresse" class="content_input" size="30" type="text" value=""/></td>
    <td width="20%">Lieu Dit :</td>
    <td width="30%"><input name="lieu_dit" class="content_input" size="30" type="text" value=""/></td>
   </tr>
   <tr>
    <td width="20%">Code Postale :</td>
    <td width="30%"><input name="code_postal" class="content_input" size="30" type="text" value=""/></td>
    <td width="20%">Ville :</td>
    <td width="30%"><input name="ville" class="content_input" size="30" type="text" value=""/></td>
   </tr>
   <tr>
    <th colspan="3">Contact :</th>
   </tr>
   <tr>
    <td width="20%"><img src="image/fixe.png" alt="Téléphone fixe" title="Téléphone fixe" height="22" width="22"/> Téléphone fixe :</td>
    <td width="30%"><input name="fixe" class="content_input" size="30" type="text" value=""/></td>
    <td width="20%"><img src="image/mobile.jpg" alt="Téléphone mobile" title="Téléphone mobile" height="22" width="22"/> Téléphone mobile :</td>
    <td width="30%"><input name="mobile" class="content_input" size="30" type="text" value=""/></td>
   </tr>
   <tr>
    <td width="20%"><img src="image/mail.png" alt="Email" title="Email" height="22" width="22"/> Adresse mail :</td>
    <td width="30%"><input name="email" class="content_input" size="30" type="text" value=""/></td>
    <td width="20%"></td>
    <td width="30%"></td>
   </tr>
   <tr>
    <th colspan="4">Informations supplémentaires :</th>
   </tr>
   <tr>
    <td width="20%">Date de Naissance :</td>
    <td width="30%" id="naissance" name="naissance">
     <input name="naissance_1" class="content_input" type="text" value="" size="2" maxlength="2"/>
     <input name="naissance_2" class="content_input" type="text" value="" size="2" maxlength="2"/>
     <input name="naissance_3" class="content_input" type="text" value="" size="4" maxlength="4"/>
     jj/mm/aaaa
    </td>
    <td width="20%"><img src="image/login.png" alt="Login" title="Login" height="22" width="22"/> Login :</td>
    <td width="30%"><input name="pseudo" class="content_input" size="30" type="text" value=""/></td>
   </tr>
   <tr>
    <td width="20%"></td>
    <td width="30%"></td>
    <td width="20%"><img src="image/password.png" alt="Mot de passe" title="Mot de passe" id="image_pwd" height="22" width="22"/> Mot de passe :</td>
    <td width="30%"><input name="password" class="content_input" size="30" type="text" value=""/></td>
   </tr>
   <tr>
    <td width="20%">Actif :</td>
    <td width="30%"><input name="actif" class="content_input" size="30" type="checkbox"/></td>
    <td width="20%"></td>
    <td width="30%"></td>
   </tr>
   <tr>
    <td width="20%">Début :</td>
    <td width="30%"><input name="debut" class="content_input" size="4" type="text" value=""/></td>
    <td width="20%"></td>
    <td width="30%"></td>
   </tr>  
   <tr>
    <td width="20%">Fin :</td>
    <td width="30%"><input name="fin" class="content_input" size="4" type="text" value=""/></td>
    <td width="20%"></td>
    <td width="30%"></td>
   </tr> 
  </table>

<!--#######################################################################################################################################-->

   <input type="hidden" name="send"/>
   <input type="submit" class="submit" value="Enregistrer"/>
        </fieldset>
 </form>
</div>