index.php

<?php
declare(strict_types = 1);

// Start session.
session_start();

// Include helper functions.
require_once 'helpers.php';

// Redirect user to login page if not authenticated.
if (!check_auth())
{
    redirect('login.php');
    return;
}

// does below line looks right? since I need to call this only button click
if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['submit']))
{
    if (!is_authorized_to_write())
    {
        alert("Your write access is revoked since another user is logged in now");
        return;
    }

    // Your code here...
    if (isset($_POST['field1']) && isset($_POST['field2']))
    {
        $data = $_POST['field1'] . '-' . $_POST['field2'] . "\r\n";
        $ret = file_put_contents('mydata.txt', $data, LOCK_EX);
        if ($ret === false)
        {
            alert("Error writing into the file");
        }
        else
        {
            alert("$ret bytes written to file");
        }
    }
    else
    {
        alert("No post data to process");
    }
}

// created function to alert in popup window
function alert($msg) {
    echo "<script type='text/javascript'>alert('$msg');</script>";
}
?>
<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Home</title>
</head>
<body>
    <div>
        <h1>Website Title</h1>
        <a href="logout.php">Logout</a>
    </div>
    <div>
        <p>Welcome back, <?=$_SESSION['user_id'] ?>!</p>
    </div>

    <form action="#" method="POST">
        <input type="text" name="field1" />
        <input type="text" name="field2" />
        <input type="submit" name="submit" value="Save Data">
    </form>

    <script src="https://code.jquery.com/jquery-3.5.1.min.js"></script>
</body>
</html>