not redirecting to other file

After clicking the login button It still stay on login.php or redirecting to same file.
I’m new to php language so I couldn’t figure out what’s wrong.

I want to redirect to navbar.php after clicking the login button
can you help me with this? thankyou

here’s the code i have

login.php

<?php
session_start();

// Check if the user is already logged in, if yes then redirect him to welcome page
if(isset($_SESSION["loggedin"]) && $_SESSION["loggedin"] === true){
  header("location: navbar.php");
  exit;
}

// Include config file
require_once "config.php";

// Define variables and initialize with empty values
$username = $password = "";
$username_err = $password_err = $login_err = "";

// Processing form data when form is submitted
  if (isset($_POST['user_login_btn'])) {
    if(isset($_POST['login_btn'])) {
      if($_SERVER["REQUEST_METHOD"] == "POST"){
        // Process the user login form
        $username = mysqli_real_escape_string($con, $_POST['username']);
        $password = mysqli_real_escape_string($con, $_POST['password']);

        $password = md5($password); // Hash the password before storing it in the database
        $query = "SELECT * FROM users WHERE username='$username' AND password='$password'";
        $results = mysqli_query($con, $query);

        if (mysqli_num_rows($results) == 1) {
        // If the login is successful, start a new session and redirect to the user dashboard
        session_start();
        $_SESSION['username'] = $username;
        $_SESSION['success'] = "You are now logged in";
        header('location: navbar.php');
        } else {
        // If the login is unsuccessful, display an error message
        array_push($errors, "Wrong username/password combination");
        }
    }
}
?>
<!DOCTYPE html>
<html>
<head>
  <title>Login</title>
</head>
<body>
  <div class="header">
    <h1>Login</h1>
  </div>
  <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
    <table>
      <tr>
        <td>Username:</td>
        <td><input type="text" name="username" required></td>
      </tr>
      <tr>
        <td>Password:</td>
        <td><input type="password" name="password" required></td>
      </tr>
      <tr>
        <td></td>
        <td><input type="submit" name="login_btn" value="Login"></td>
      </tr>
    </table>
  </form>
</body>
</html>