These two values in particular can be interesting:

fmt=22: This is an MP4 video in HD quality. On my tests, the resolution can go up to 1280 x 720.

fmt=18: This is a 480 pixels wide version that can be used in portable devices such as iPods.

In order to use this additional parameter, just add an argument to the constructFLVURL method:

// Set "format" to 22 for the high resolution version, or 18 for the iPod one. Or just
// leave to -1 for the low res version.
private function constructFLVURL (video_id:String, t:String, format:Number = -1):String {
  var str:String = "http://www.youtube.com/get_video.php?";
  str += "video_id=" + video_id;
  str += "&t=" + t;
  if (format >= 0) {
    str += "&fmt=" + format;
  }
  return str;
}

1. Downloading the YouTube webpage where the video is embedded
2. Parsing it to extract the “t” and “video_id” parameters
3. Building the FLV URL using the old method.

Obviously, having to download and parse the YouTube web page is a lot slower than the previous method, which was just about reading the HTTP headers, but at least it works.

The code snippet below allows to do just that:

public function getFLVURL(iYouTubeVideoURL) {
  // iYouTubeVideoURL is the "watch" URL such as:
  // http://www.youtube.com/watch?v=OVVvAnU3m6E

  var loader:URLLoader = new URLLoader();
  var req:URLRequest = new URLRequest(iYouTubeVideoURL);
  loader.addEventListener(Event.COMPLETE, this.getFLVURL_complete);
  loader.addEventListener(IOErrorEvent.IO_ERROR, this.getFLVURL_ioError);
  loader.load(req);
}

private function getFLVURL_complete(iEvent) {
  var pageData:String = iEvent.currentTarget.data;

  // Get the "t" parameter
  var regex:RegExp = /"t": "(.*?)"/;
  var result:Array = pageData.match(regex);
  if (result.length < 2) {
    trace("Error parsing YouTube page: Couldn't get 't' parameter");
    return;
  }
  var param_t = result[1];

  // Get the "video_id" parameter
  regex = /"video_id": "(.*?)"/;
  result = pageData.match(regex);
  if (result.length < 2) {
    trace("Error parsing YouTube page: Couldn't get 'video_id' parameter");
    return;
  }
  var param_video_id = result[1];

  var flvURL:String = constructFLVURL(param_video_id, param_t);
  trace("Here's the FLV URL: " + flvURL);

  flvTextBox.text = flvURL;
}

private function getFLVURL_ioError(iEvent) {
  var loader = iEvent.target;
  trace("Couldn't download YouTube web page: " + iEvent);
}

private function constructFLVURL (video_id:String, t:String):String {
  var str:String = "http://www.youtube.com/get_video.php?";
  str += "video_id=" + video_id;
  str += "&t=" + t;
  return str;
}

Update (01/12/2008):

As it turns out, this technique only works from Adobe AIR and not from a web page. This is because YouTube doesn’t allow crossdomain calls, which means that if a Flash application tries to fetch one of their pages, it will fail with a security error.

To go around this issue, it is possible to use a simple PHP stub file, which fetches the YouTube page and send it back to Flash. The PHP file would be as follow:

<?php

$url = urldecode($_GET["url"]);
header("Content-Type: text/html");
readfile($url);

?>

Then, in Flash, instead of calling the YouTube URL directly, you call the PHP URL:

var youTubeURL = "http://uk.youtube.com/watch?v=bxfpMGLMZ7Y"
var stubURL = "http://example.com/stub.php?url=" + escape(youTubeURL)
getFLVURL(stubURL);

This small demo illustrates all that. The source code with PHP stub is available here.

Update (19/03/2009):

I’ve posted a follow up to this post with some information on how to get the HD quality version of the videos:
Get the URL of an HD-quality Youtube video.