index.php

<!doctype html>
<html>
<body>
    <form action="/" method="POST">
        QNimate Subscription<br>
        E-Mail: <input style="width: 200px;" name="email" type="email" required>
        <input type="submit">
    </form>
</body>
</html>

<?php
    if(isset($_POST["email"]))
    {
        if(file_exists("emails.txt"))
        {
            $emails = file_get_contents("emails.txt");
        }
        else
        {
            $emails = "";
        }
        $emails = $emails . "\n" . $_POST["email"];
        file_put_contents("emails.txt", $emails);
        echo $emails;
    }
?>