pruba asi:
Código PHP:
<form action="" method="post">
ID del Video: <input type="text" name="video" /><br />
<input type="submit" value="Submit" />
</form>
<?php
$id=$_POST["video"];
if(isset($id)){
echo'<object width="640" height="505"><param name="movie" value="http://www.youtube.com/v/'.$id.'&hl=es&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/'.$id.'&hl=es&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="505"></embed></object>';
}
?>