$string = " fasfasd  http://webarto.com   fasfsafa";

echo preg_replace("#http://([\S]+?)#Uis", '<a rel="nofollow" href="http://\\1">\\1</a>', $string);

输出：

 fasfasd  <a rel="nofollow" href="http://webarto.com">webarto.com</a>   fasfsafa