Bu şimdiye kadar var budur:

<?php

 $content = "word1 http://www.youtube.com/watch?v=yqfKe-67foQ&feature=related word2 http://www.youtube.com/watch?v=2vq7gDEn99Y&feature=related word3 http://www.youtube.com/watch?v=nW5HxgMYRto\nhttp://www.youtube.com/watch?v=8Uc2lpH0iZ0&feature=fvhl";
 $pattern = '/http:\/\/www\.youtube\.com\/watch\?(.*)v=([a-zA-Z0-9_\-]+)(\S*)/i';
 $replace = '<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/$2&amp;hl=en_US&amp;fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/$2&amp;hl=en_US&amp;fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>';

 $content = preg_replace($pattern, $replace, $content);
 echo $content;

?>

Bu işe yaramazsa neden ben dürüst hiçbir fikrim yok. Bazı yardım mutluluk duyacağız. Teşekkürler.