[PHP/SQL] INSERT mit PDO prepared funktioniert nicht.

if(empty($_POST['username']) || empty($_POST['email']) || empty($_POST['password'])){
    echo "Signup form isn't filled.";
    exit();
}else{
    // Set variables
    $username = $_POST['username'];
       $email = $_POST['email'];
    $password = $_POST['password'];


    // Prepare query
    $stmt = $pdo->prepare("SELECT * FROM users WHERE email = ? OR username = ?");
    $stmt->execute([$email,$username]);
    if($stmt->rowCount() !== 0){
        echo 'Email or username is taken.';
        exit();
    }else{
        // FIX: Doesn't insert anything into the DB.
        $stmt = $pdo->prepare("INSERT INTO users (email,password,username) VALUES (?,?,?)");
        $stmt->execute([$email,$password,$username]);
        echo 'User has been created.';
        exit();
    }
}

if(empty($_POST['username']) || empty($_POST['email']) || empty($_POST['password'])){
   echo 'Signup form isn\'t filled.';
}else{

   // Set variables
   // HIER TROTZDEM BESSER PRÜFEN. Prepared-Statements schützen nicht vor XSS.  
   $username = $_POST['username'];
   $email = $_POST['email'];
   // Und warum wird hier ein Klartext-Passwort gespeichert? ARGH ^^ Erinnert mich bitte daran, mich nie bei irgendeinem System von BloodSteam zu registrieren. ^^
   $password = $_POST['password'];

   // Prepare query
   $stmt = $pdo->prepare('SELECT 1 FROM users WHERE email = :email OR username = :username');
   $stmt->bindValue(':email', $email, PDO::PARAM_STR);
   $stmt->bindValue(':username', $username, PDO::PARAM_STR);
   if ( $stmt->execute() ) {
      if ( (int)$pdo->errorCode() ) {
         echo 'Fehler ' . print_r($pdo->errorInfo(), true);
      } else {
         if ($stmt->rowCount() !== 0) {
            exit('Email or username is taken.');
         } else {
            // FIX: Doesn't insert anything into the DB.
            $stmtInsert = $pdo->prepare('INSERT INTO users (email,password,username) VALUES (:email,:password,:username)');
            $stmtInsert->bindValue(':email', $email, PDO::PARAM_STR);
            $stmtInsert->bindValue(':password', $password, PDO::PARAM_STR);
            $stmtInsert->bindValue(':username', $username, PDO::PARAM_STR);
            if ( $stmtInsert->execute() ) {
               if ( (int)$pdo->errorCode() ) {
                  echo 'Fehler ' . print_r($pdo->errorInfo(), true);
               } else {
                  echo 'User has been created.';
               }
            }
         }
      }
   }
}
exit;

$stmt = $pdo->prepare("INSERT INTO users (email,password,username) VALUES (?,?,?)");

$stmt = $pdo->prepare("INSERT INTO users (email,password,username) VALUES (:email, :password, :username)");

$stmt = $pdo->prepare("INSERT INTO users (email,password,username) VALUES ('abc','123','f121212')");

$stmtInsert = $pdo->prepare('INSERT INTO users (email,password,username) VALUES (:email,:password,:username)');
            $stmtInsert->bindValue(':email', $email, PDO::PARAM_STR);
            $stmtInsert->bindValue(':password', $password, PDO::PARAM_STR);
            $stmtInsert->bindValue(':username', $username, PDO::PARAM_STR);
            if ( $stmtInsert->execute() ) {
               if ( (int)$pdo->errorCode() ) {
                  echo 'Fehler ' . print_r($pdo->errorInfo(), true);
               } else {
                  echo 'User has been created.';
               }
            }
        exit;

include_once 'dbh.php';

class Users extends Dbh
{
    private $sql = 'SELECT id,username FROM users';

    function __construct()
    {
        parent::__construct();
    }

    public function getUsers(){
        return $this->conn->query($this->sql);
    }
}

$users = new Users();

$result = $users->getUsers();
echo var_dump($result);

class Dbh
{
    private $host = '127.0.0.1';
    private $port = '3306';
    private $username = 'root';
    private $password = '123';
    private $dbname = 'loginsystemdb';
    private $charset = 'utf8mb4';

    protected $conn;

    function __construct()
    {
        try {
            $this->conn = new PDO("mysql:host=$this->host;port=$this->port;dbname=$this->dbname;charset=$this->charset", $this->username, $this->password);
            $this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
            return $conn;
        } catch (PDOException $e) {
            echo 'Mysql Connection failed: '.$e->getMessage();
        }
    }
}

echo var_dump($result->fetchall());

public function insert($email,$password,$username)
    {
        $this->bind(':email', 'hello@kevin.com');
        $this->bind(':password', 'pro420');
        $this->bind(':username', 'XxPROxX');
        $this->execute();
    }

public function insert($email,$password,$username)
    {
        $this->query('INSERT INTO users (email, password, username) VALUES (:email, :password, :username)');
        $this->bind(':email', 'hello@kevin.com');
        $this->bind(':password', 'pro420');
        $this->bind(':username', 'XxPROxX');
        $this->execute();
    }

return { 'type' : 'signup', 'text' : 'Erfolgreich Zugestellt'}

<br /> <b>Fatal error</b>:  Uncaught Error: Call to a member function prepare() on null in dbc.php:51 Stack trace:
#0 signup.php(12): Database->query('SELECT email,us...') 
#1 router.php(37): Signup->addNewUser('damian@q.com', '$2y$10$xEf6v.7a...', 'damian') 
#2 {main}   thrown in <b>dbc.php</b> on line <b>51</b><br />

    public function query($query)
    {
        $this->stmt = $this->dbh->prepare($query);
    }