function is_url_ok($url) {
    $a=parse_url($url);

    $scheme=strtolower($a["scheme"]);
    if($scheme!='http') {return false;}

    $host=strtolower($a["host"]);

    $port=$a["port"];
    if($port=="") $port=80;

    $path=$a["path"];
    if($path=="") $path="/";

    $query=$a["query"];
    if($query!="") $query="?".$query;

    $reassembled_url="http://".$host;
    if($port!=80) $reassembled_url.=":$port";
    $reassembled_url.=$path.$query;

    $sock=fsockopen($host,$port,$errno,$errstr,20 /* timeout sec */);
    if(!$sock) {return false;};

    fputs($sock,"HEAD $path$query HTTP/1.0\r\nHost: $host\r\nReferer: http://$host/\r\nConnection: Close\r\n\r\n");

    $dt="";
    do {
      $dt.=fread($sock,512);
      $st=socket_get_status($sock);
    }
    while($st["unread_bytes"]);
    fclose($sock);

    //parse HTTP headers
    $lines=explode("\n",$dt);
    $resp=trim($lines[0]);
    if(!preg_match("~200 OK$~i",$resp)) return false;
    return true;
}