Код:
<?php
if(isset($_POST['x'])) {
        $yo= ''.base64_decode($_POST['x']).'';
}
?>
<body>
<br />
  <form action="<?=$_SERVER['php_self']?>" method="post">
<div align="center">  <textarea name="x" cols="60" rows="25" id="x"><?=@$yo?></textarea>
  <br />
      <input type="submit" value="Decode" />
      <br />
</div>
</body>