if(isset($_POST['email']) && !empty($_POST['email'])){
        $email = $_POST['email'];
        // 1. file openen in juist module
        $file = fopen("email.txt","a");
        // 2. wegschrijven emailadres (\n, \r)
        fwrite($file,$email . "\n");
        // 3. file sluiten
        fclose($file);
    }