<?php if (!$_POST) { ?>

<form action="" method="post">
YouTube URL:  <input type="text" name="url" /><br />
<input type="submit" name="submit" value="Submit me!" />
</form>
<?php }  ?>

<?php

error_reporting(E_ALL);

if ($_POST) {
    $request_url = 'http://www.youtube.com/oembed?url='.urlencode($HTTP_POST_VARS['url']).'&format=xml';
    $xml = simplexml_load_file($request_url) or die("feed not loading");
    echo $xml->html[0];
}

?>