<form method='post' action='?='>
    <table>
    <tr>
        <td>
            <textarea name='encode' cols='20' rows='20'><?php
            $data =  (htmlentities(isset($_POST['encode']) ?  $_POST['encode'] : "" ));
            echo base64_encode($data);
            ?></textarea>
        </td>

        <td>
            <textarea name='decode' value='Decode Box' cols='20' rows='20'><?php
            $data2 =  (htmlentities(isset($_POST['decode']) ?  $_POST['decode'] : "" ));
            echo base64_decode($data2);
            ?></textarea>
        </td>
    </tr>
    </table>
    <br>
    <input type='submit'>
</form>