PHP / mySQL -database connection successful, but I can't write or display data

DB connection is successfull, but I can't write data to DB neather display it to html/php file. I use 000webhost with mySQL. This is project for university, web programming. I must make website (portfolio) but it has to include php and database. Can you help me to find errors because I'm desperate, I tried everything so far. also, my login doesn't work, I feel like I haven't connected to the database at all even though it says otherwise.

THIS IS ADMIN PANEL (When login is successfull, it has to redirect to admin-panel)

include 'connect.php';

$sql = "SELECT * FROM projekt_web;";
$result = mysqli_query($connection, $sql);
$resultCheck = mysqli_num_rows($result);

if ($resultCheck > 0) {
while($row = mysqli_fetch_assoc($result)) {
    echo "ID: " . $row["id"]. " Name: " . $row["firstName"]." E-mail: " . $row["email"]. "Poruka: " . $row["poruka"]. "<br>";
}
} else {
    echo "U tablici nema zapisa.";
}

mysqli_close($connection);

THIS IS ADMIN LOGIN:

    include 'connect.php';

  if($_SERVER["REQUEST_METHOD"]==="POST"){
    $username=$_POST["username"];
    $password=$_POST["password"];
  
    $sql = "SELECT * FROM $db WHERE username='".$username."' AND password='".$password."'; ";
  
    $result = mysqli_query($connection, $sql);
  
    $row = mysqli_fetch_array($result);
  
    if($row["username"]=="admin" and $row["password"]=="admin"){
      $_SESSION["username"]=$username;
      header("location: admin-panel.php");
    }else{
      echo '<script>alert("Korisničko ime i/ili lozinka nisu ispravni. Molimo unesite ispravne podatke.")</script>';
    }
  }



  mysqli_close($connection);

DB CONNECTION:

 $host = "localhost";
  $user = "id17984309_projekt_web278";
  $password = "\J10L%_4OVMCo@ci";
  $db = "id17984309_projekt_web";

  $connection = mysqli_connect("$host", "$user", "$password", "$db");

  if (!$connection) {
    die("Connection failed: " . mysqli_connect_error());
  }
  echo "Connected successfully.";


THIS IS index.php where is login form and php code:

if(isset($POST['submit'])){
    $firstName = $_POST['firstName'];
    $lastName = $_POST['lastName'];
    $email = $_POST['email'];
    $poruka = $_POST['poruka'];
    
    $sql = "INSERT INTO $db (firstName, lastName, email, poruka) VALUES ('$firstName', '$lastName', '$email', '$poruka');";
    
    $result = mysqli_query($connect, $sql);
    
    if($result)
    {
        $message = "Vaša poruka je uspješno poslana!";
        echo "<script type='text/javascript'>alert('$message');</script>";
    }

    mysqli_close($connection);
}

image mySQL database (2 tables, first for login and second for messages from future users from portfolio)

source https://stackoverflow.com/questions/70173545/php-mysql-database-connection-successful-but-i-cant-write-or-display-data