.navbar{
    position: fixed;
    ...
}


<nav class="navbar navbar-inverse">
...
</nav>


<form  method="post"; style="font-family:verdana; text-align:center">
   ...
 </form>


$secret = "6LfzPTcUAAAAAK7rR7fq9C_XOj1K73kEweAJskxO";
$sprawdz = file_get_contents('https://google.com/recaptcha/api/siteverify?secret='.$secret.'&response='. $_POST['g-recaptcha-response']);
$odpowiedz = json_decode($sprawdz);
 ...
<center>
<div class="g-recaptcha" data-sitekey= "6LfzPTcUAAAAAHxzIJl_8KREV3QpalPZPpE4jUV3">
</div>
</center>




try
        {
            $polaczenie = new mysqli($host, $db_user, $db_password, $db_name);
...
if($polaczenie->query("INSERT INTO uzytkownicy VALUES(NULL, '$nick','$haslo_hash', '$email',0,'$klucz_hash')"))
...
     }


<?php
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\Exception;
require 'C:/xampp/phpmailer/src/Exception.php';
require 'C:/xampp/phpmailer/src/PHPMailer.php';
require 'C:/xampp/phpmailer/src/SMTP.php';
  ...
?>


<?php
...
$klucz_hash = password_hash($nick.$email.$haslo, PASSWORD_DEFAULT);
...
?>


$mail = new PHPMailer(true);
                    try {
                     $mail->isSMTP();
                     $mail->Host = 'smtp.gmail.com';
                     $mail->SMTPAuth = true;
                     $mail->Username = "testsejf@gmail.com";
                     $mail->Password = "Grazynka123";
                     $mail->SMTPSecure = 'ssl';
                     $mail->Port = 465;

                     $mail->CharSet = "UTF-8";
                     $mail->setLanguage('pl', 'C:/xampp/phpmailer/language');

                     $mail->setFrom('testsejf@gmail.com', 'Smart safe');
                     $mail->addAddress($email,$nick);
                     $mail->isHTML(true);
                     $mail->Subject = $temat;
                     $mail->Body = $tresc;
                     $mail->AltBody = 'By wyświetlić wiadomość należy skorzystać z czytnika obsługującego wiadomości w formie HTML';

                     $mail->send();


<?php
...
    $dbconnect->query("INSERT INTO sensor (content) VALUES ('".$_GET["value"]."')");

?>


<nav class="navbar navbar-inverse">
        <div class="container-fluid">
          <div class="navbar-header">
            <a class="navbar-brand">Smart Sejf</a>
          </div>
          <ul class="nav navbar-nav">
            <li class="active"><a href="index.html">Home</a></li>
            <li><a href="produkty.html">Products</a></li>
            <li><a href="logowanie.html">Sign up</a></li>
            <li><a href="kontakt.html">Contact</a></li>
          </ul>
        </div>
      </nav>