How to redirect after successful registration?

Question

PHP

There are three pages index.php, register.php and poup.php
register.php

<?php include_once 'poup.php'; ?>
    <div id="content">  
        <form method="post" action="" id="formregistr">
            <input type="text" name="username" placeholder="  Имя Пользователя" contextmenu="" minlength="4" required=""><p />
            <input type="text" name="login" placeholder="  Логин" minlength="4" required=""><p />
            <input type="password" name="password" placeholder="  Пароль" minlength="2" required><p />
            <input type="password" name="r_password" placeholder="  Подтверждение Пароля" minlength="2" required=""><p />
            <input type="submit" name="submit" value="Регистрация"><p />
                <?php echo $a_not_error; ?>
                <?php echo $a_error; ?>
                <?php echo $a_error_duble; ?>
                <?php echo $a_error_lang; ?>
        </form>
    </div>

popup.php

<?php include_once 'header.php'; ?>
<?php
$connect = mysql_connect('localhost', 'root', '') or die(mysql_error());
mysql_select_db('test');

    if(isset($_POST['submit'])) {
        $username = $_POST['username'];
        $login = $_POST['login'];
     if(preg_match("/[^A-Za-z0-9_-]/u", $_POST['login'])) {
        $a_error_lang = "Только латинские буквы и цифры";
        return include_once 'registr.php';
        }
     
    $password = $_POST['password'];
    $r_password = $_POST['r_password'];
       
    $result =  mysql_query("SELECT id FROM registr WHERE login='$login'");
    $user_duble = mysql_fetch_array($result);
    
    if ($user_duble == TRUE) {
           $a_error_duble = 'Пользователь с таким логином зарегистрирован';
           return include_once 'registr.php';
        } 
           
    if($password == $r_password) {
           $password = md5($password);
           $result_2 = mysql_query("INSERT INTO registr VALUES('id',
                   '$username','$login','$password')") or die(mysql_error());
           $a_not_error = "Вы успешно зарегистрировались!";
        }
    else $a_error = "Несовпадение паролей"; 
    }
?>

And so it would be desirable that after successful registration "You have successfully registered!" and then redirected to the index.php page
php just started learning so don't swear too much =)

Answer 1

<?php if (!empty($a_not_error)) { ?>
    <p>Вы успешно зарегистрировались! Через 5 секунд будет произведено перенаправление на главную страницу</p>
    <script> window.setTimeout(function() { window.location = 'index.php'; }, 5000) </script>
<?php } ?>