Bootstrap MODAL REGISTRATION not working

I have this code

  <form id="form" role="form" method="post" action="" autocomplete="off" enctype="multipart/form-data">

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span></button>
        <h4 class="modal-title" id="myModalLabel">Modal title</h4>
      </div>
      <div class="modal-body">




                <?php
                // Verificar erros
                if(isset($error)){
                    foreach($error as $error){
                        echo '<p class="register-errors">'.$error.'</p>';
                    }
                }

                // Mensagem de sucesso
                if(isset($_GET['action']) && $_GET['action'] == 'joined'){
                    echo "<h2 class='register-sucess'>Registration successful, please check your email to activate your account.</h2> ";
                }
                ?>


    <input type="text" name="username" id="username" class="register-username" placeholder="Username" value="<?php if(isset($error)){ echo $_POST['username']; } ?>" tabindex="1" maxlength="12">


    <br>
    <img id="uploadPreview1" src="../cdn/css/images/no_image.jpg" width="130" height="130"/>

    <input id="uploadImage1" type="file" name="img[]" onchange="PreviewImage(1);" />
           <label for="uploadImage1" >Add profile picture</label>


    <br>



    <input type="email" name="email" id="email" class="register-email" placeholder="Email Address" value="<?php if(isset($error)){ echo $_POST['email']; } ?>" tabindex="2">

    <br>

    <input type="password" name="password" id="password" class="register-password" placeholder="Password" tabindex="3">
    <input type="password" name="passwordConfirm" id="passwordConfirm" class="register-confirmpassword" placeholder="Confirm Password" tabindex="4">

    <br>


    <br>







<?  



if(isset($_POST['submit'])){


        if(strlen($_POST['username']) < 3){
            $error[] = 'Username is too short.';
        } else {
            $stmt = $db->prepare('SELECT username FROM members WHERE username = :username');
            $stmt->execute(array(':username' => $_POST['username']));
            $row = $stmt->fetch(PDO::FETCH_ASSOC);

            if(!empty($row['username'])){
                $error[] = 'Username provided is already in use.';
            }
        }

        if(strlen($_POST['password']) < 3){
            $error[] = 'Password is too short.';
        }

        if(strlen($_POST['passwordConfirm']) < 3){
            $error[] = 'Confirm password is too short.';
        }

        if($_POST['password'] != $_POST['passwordConfirm']){
            $error[] = 'Passwords do not match.';
        }

        if(!filter_var($_POST['email'], FILTER_VALIDATE_EMAIL)){
            $error[] = 'Please enter a valid email address';
        } else {
            $stmt = $db->prepare('SELECT email FROM members WHERE email = :email');
            $stmt->execute(array(':email' => $_POST['email']));
            $row = $stmt->fetch(PDO::FETCH_ASSOC);

            if(!empty($row['email'])){
                $error[] = 'Email provided is already in use.';
            }

        }


        if(!isset($error)){

            // Proteger a password
            $hashedpassword = $user->password_hash($_POST['password'], PASSWORD_BCRYPT);

            // Criar código de activação
            $activasion = md5(uniqid(rand(),true));

            try {

        $file       = $_FILES['img'];
        $numFile    = count(array_filter($file['name']));

        //PASTA
        $folder     = '../cdn/uploads/avatars/';

        //REQUISITOS
        $permite    = array('image/jpeg', 'image/png');
        $maxSize    = 1024 * 1024 * 1;

        //MENSAGENS
        $msg        = array();
        $errorMsg   = array(
        1 => 'O arquivo no upload é maior do que o limite definido em upload_max_filesize no php.ini.',
        2 => 'O arquivo ultrapassa o limite de tamanho em MAX_FILE_SIZE que foi especificado no formulário HTML',
        3 => 'o upload do arquivo foi feito parcialmente',
        4 => 'Não foi feito o upload do arquivo'
            );

    if($numFile <= 0){
        echo '<script>alert(asd);</script>';
    }
    else{
        for($i = 0; $i < $numFile; $i++){
        $name   = $file['name'][$i];
        $type   = $file['type'][$i];
        $size   = $file['size'][$i];
        $error  = $file['error'][$i];
        $tmp    = $file['tmp_name'][$i];

        $extensao = @end(explode('.', $name));
        $avatar = rand().".$extensao";

        if($error != 0)
            $msg[] = "<b>$name :</b> ".$errorMsg[$error];
        else if(!in_array($type, $permite))
            $msg[] = "<b>$name :</b> Erro imagem não suportada!";
        else if($size > $maxSize)
            $msg[] = "<b>$name :</b> Erro imagem ultrapassa o limite de 1MB";
        else{

        if(move_uploaded_file($tmp, $folder.'/'.$avatar)){


                $stmt = $db->prepare('INSERT INTO members (username,password,email,active,avatar) VALUES (:username, :password, :email, :active, :avatar)');
                $stmt->execute(array(
                    ':username' => $_POST['username'],
                    ':password' => $hashedpassword,
                    ':email' => $_POST['email'],
                    ':active' => $activasion,
                    ':avatar' => $avatar
                ));


     }else{
     $msg[] = "<b>$name :</b> Desculpe! Ocorreu um erro...";

       }
      }
     }
     }

                // Inserir dados na base de dados

                $id = $db->lastInsertId('memberID');

                // Estrutura do email
                $to = $_POST['email'];
                $subject = " Registration Confirmation";
                $body = "Thank you for registering at site.\n\n To activate your account, please click on this link:\n\n ".DIR."activate.php?x=$id&y=$activasion\n\n\n Regards,\n Administration \n\n";
                $additionalheaders = "From: <".SITEEMAIL.">\r\n";
                $additionalheaders .= "Reply-To: ".SITEEMAIL."";
                mail($to, $subject, $body, $additionalheaders);

                header('Location: register.php?action=joined');
                exit;

            } catch(PDOException $e) {
                $error[] = $e->getMessage();
            }

        }

    }


?>

      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        <input type="submit" name="submit" value="Register" class="btn btn-primary" tabindex="5">
        </form>
      </div>
    </div>
  </div>
</div>

What happens is this, when I give Submit it does not add anything in the database, only makes a refresh on the page. And also like that mistakes appear in Ajax / jQuery without having to refresh the page after submit. Both email errors, user etc .. As the avatar so there mistakes but do not appear: ss

In short, I wanted help to FIX the part of not going anything to the database and show all errors including Avatar. And all of them in ajax without having to refresh the page ..

Please, anyone help me, it's really urgent :ss