login.php

<?php

// Database connection
$conn = mysqli_connect("localhost", "root", "", "example_db");

// Login logic
if(isset($_POST['login'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    // Check if user is admin
    $query = "SELECT * FROM users WHERE username='$username' AND password='$password' AND role='admin'";
    $result = mysqli_query($conn, $query);
    $isAdmin = mysqli_num_rows($result) > 0;

    // Check if user is student
    $query = "SELECT * FROM users WHERE username='$username' AND password='$password' AND role='student'";
    $result = mysqli_query($conn, $query);
    $isStudent = mysqli_num_rows($result) > 0;

    if($isStudent || $isAdmin) {
        // Start session and store role
        session_start();
        $_SESSION['role'] = $isAdmin ? 'admin' : 'student';

        // Redirect to appropriate page
        if($isAdmin) {
            header("Location: admin_dashboard.php");
        } else {
            header("Location: student_dashboard.php");
        }
    } else {
        echo "Invalid login credentials";
    }
}

?>

<!DOCTYPE html>
<html>
<head>
    <title>Login and Register Form</title>
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">

</head>
<body>
    <h1>Login</h1>
    <form method="POST">
        <input type="text" name="username" placeholder="Username" required><br>
        <input type="password" name="password" placeholder="Password" required><br>
        <button type="submit" name="login">Login</button>
    </form>
</body>
</html>
Please add password hashing. You should not be storing passwords as plain text.