w8_view_startpage.php:

<div id='signin' class='modal-window'>
        <h2 style='text-align:center'>Sign In</h2>
        <br>
        <form method='post' action='w4_controller.php'>
            <input type='hidden' name='page' value='StartPage'></input>
            <input type='hidden' name='command' value='SignIn'></input>
            <label class='modal-label'>Username:</label>
            <input type='text' name='username' placeholder='Enter username' required></input>
            <br>
            <label class='modal-label'>Password:</label>
            <input type='password' name='password' placeholder='Enter password' required></input>
            <br>
            <br>
            <button type='submit' value='Submit'>Submit</button>
            <button type='reset' value='Reset'>Reset</button>
            <button id='cancel-signin' type='cancel' value='Cancel'>Cancel</button>
        </form>
    </div>

    <div id='join' class='modal-window'>
        <h2 style='text-align:center'>Join</h2>
        <br>
        <form method='post' action='w4_controller.php'>
            <input type='hidden' name='page' value='StartPage'></input>
            <input type='hidden' name='command' value='Join'></input>

            <label class='modal-label'>Username:</label>
            <input type='text' name='username' placeholder='Enter username' required></input>
            <br>
            <label class='modal-label'>Password:</label>
            <input type='password' name='password' placeholder='Enter password' required></input>
            <br>
            <label class='modal-label'>Email:</label>
            <input type='text' name='email' placeholder='Enter email address' required></input>
            <br>
            <br>
            <button type='submit' value='Submit'>Submit</button>
            <button type='reset' value='Reset'>Reset</button>
            <button id='cancel-join' type='cancel' value='Cancel'>Cancel</button>
        </form>
    </div>



w4_controller.php

<?php
require('w4_model.php');  // This file includes some routines to use DB.
               // E.g., is_valid() to check the validity of username and password.
//...

// When commands come from StartPage
if ($_POST['page'] == 'StartPage')  // Check the page value
{
    $command = $_POST['page'];
    switch($command) {  // When a command is sent from the client
        case 'SignIn':  // With username and password
//          if (there is an error in username and password) {
            if (!is_valid($_POST['username'], $_POST['password'])) {  // This function is defined in Model.
                include('w4_view_startpage.php');  // Include the start page
            } else
                include('w4_view_mainpage.php')  // Include the main page
            exit();

        case 'Join':  // With username, password, email, some other information
            //...
            exit();
        //...

        default:
            exit();
    }
}

// When commands come from 'MainPage'
//   Not implemented yet
else {
    //...
}
?>