вот накатал на PHP
<?
if(isset($_POST['cod']))
{
$enc=htmlspecialchars($_POST['cod']);
$cod=urlencode($_POST['cod']);
}else if(isset($_POST['enc']))
{
$cod=$enc;
$enc=urldecode($_POST['enc']);
}else
{
$enc="Your XSS";
$cod="";
}
?>
<table border=0 align=center><tr><td><fieldset><legend><b>URL Code/Encode</b></legend><form method=post>
<b>Text:</b><br><textarea cols=40 rows=3 name=cod><?print $enc;?></textarea><br><INPUT type=Submit value=' Code '><INPUT type='reset' value=Clear></form>
<form method=post><b>URL:</b><br><textarea cols=40 rows=3 name=enc><?print $cod;?></textarea><br><INPUT type=Submit value='Encode '><INPUT type='reset' value=Clear></form>
</fieldset></td></tr></table>