It's worth pointing out that if you are using AJAX and need to encode strings that are being sent to a PHP application, you may not need to decode them in PHP.
<?php
echo stripslashes(nl2br($_POST['message']));
?>
Will properly output a message sent with the javascript code if the message is encoded:
message = encodeURIComponent(message)
And is sent with an AJAX POST request with the header:
ajaxVar.setRequestHeader('Content-type', 'application/x-www-form-urlencoded')