A small php function that transforms a youtube url in its embed script.

function make_youtube_embed_from_url($url)
{
    $url = str_replace('?v=','/v/',$url);
    $out = '<object width=\"425" height=\"350\"><param name=\"movie\" value=\"'. $url .'\"></param><param name=\"wmode\" value=\"transparent\"></param><embed src=\"'. $url .'\" type=\"application/x-shockwave-flash\" wmode=\"transparent\" width=\"425\" height=\"350\"></embed></object>';
    return $out;   
}