Código PHP:
$youtube_url = "http://www.youtube.com/watch?v=p4kVWCSzfK4";
if( strstr($youtube_url, "&") ){
echo str_replace("http://www.youtube.com/watch?v=" , "http://www.youtube.com/embed/" , current(explode("&", $youtube_url)));
}else{
echo str_replace("http://www.youtube.com/watch?v=" , "http://www.youtube.com/embed/" , $youtube_url);
}
Solucionado, gracias :)