Fetch YouTube highest thumbnail resolution

I want to get youtube highest thumbnail "maxresdefault.jpg"

Like this one

http://i.ytimg.com/vi/Cj6ho1-G6tw/maxresdefault.jpg

I'm using this simple php code

<?php$youtub_id = "Cj6ho1-G6tw";echo "http://i.ytimg.com/vi/".$youtub_id."/maxresdefault.jpg";?>

The problem with the code above is there is videos like this one http://youtu.be/VGazSZUYyf4NOT HD

And the result is gray small 404 image of youtube

http://i.ytimg.com/vi/VGazSZUYyf4/maxresdefault.jpg

So how to get the highest youtube thumbnail so if "maxresdefault" not available get the next big thumbnail "hqdefault", if not get the next "mqdefault" etc ...

I tried to use gdata youtube but either way the video hd or not "maxresdefault" not showing.