$urls=array("http://google.com/1.html","google.com/2.html","http://yahoo.com","yahoo.com/3.html");

foreach ($urls as $url)
{
list($domain,)=explode("/",strstr($url,"http://")?str_replace("http://","",$url):$url);
$domains[$domain]?null:$domains[$domain]=$url;
}

print_r($domains);

function strip_url_down($the_url) {
ereg("^http://w*\.*(.+)\\\?",$the_url,$regs);
$new = explode("/",$regs[1]);
$final_url = $new[0];
$levels = explode(".",$final_url);
if(sizeof($levels)==4) $final_url = $levels[1].".".$levels[2].".".$levels[3];
elseif(sizeof($levels)==3) $final_url = $levels[1].".".$levels[2];
elseif(sizeof($levels)==2) $final_url = $levels[0].".".$levels[1];
return $final_url;
}