Simple md5 encrypt.

keygen.php

<?php
$data = $_POST["data"];
$is_data = isset($data);
?>
<form method="post">
    <label> Key
    <?php if ($is_data): ?>
    <input name="data" type="text" value="<?= md5($data) ?>" size="30" />
    <?php else: ?>
    <input name="data" type="password" size="30" />
    <?php endif ?>
    </label>
    <input type="submit" value="Encrypt" />
</form>