session_start();

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

require_once "config.php";

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

// Processing form data when form is submitted
if($_SERVER["REQUEST_METHOD"] == "POST"){

    // Validate credentials
    if(empty($username_err) && empty($password_err)){
    // Prepare a select statement
        $sql = "SELECT id, username, password FROM users WHERE username = ?";

        if($stmt = mysqli_prepare($link, $sql)){
    // Bind variables to the prepared statement as parameters
            mysqli_stmt_bind_param($stmt, "s", $param_username);

    // Set parameters
            $param_username = $username;

    // Attempt to execute the prepared statement
            if(mysqli_stmt_execute($stmt)){
    // Store result
                mysqli_stmt_store_result($stmt);

    // Check if username exists, if yes then verify password
                if(mysqli_stmt_num_rows($stmt) == 1){
    // Bind result variables
                    mysqli_stmt_bind_result($stmt, $id, $username, $hashed_password);
                    if(mysqli_stmt_fetch($stmt)){
                        if(password_verify($password, $hashed_password)){
                        // Password is correct, so start a new session
                            session_start();

                        // Store data in session variables
                            $_SESSION["loggedin"] = true;
                            $_SESSION["id"] = $id;
                            $_SESSION["username"] = $username;

                        // Redirect user to welcome page
                            header("location: welcome.php");
                        } else{
                        // Display an error message if password is not valid
                            $password_err = "The password you entered was not valid.";
                        }
                    }
                } else{
                // Display an error message if username doesn't exist
                    $username_err = "No account found with that username.";
                }
            } else{
                echo "Oops! Something went wrong. Please try again later.";
            }
        }

    // Close statement
        mysqli_stmt_close($stmt);
    }

// Close connection
    mysqli_close($link);
}

<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
<div class="form-group <?php echo (!empty($username_err)) ? 'has-error' : ''; ?>">

    <H2>Username:</H2>
    <input type="text" name="username" class="form-control" value="<?php echo $username; ?>">
    <H2>Password:</H2>
    <input type="password" name="password" class="form-control">
    <div class="form-group">
        <input type="submit" class="btn btn-primary" value="Login" style="width:80px;height:42px;font-size:18px;">
    </div>
</div>
</form>