function buildPhotoURL ($photo, $size = "Medium")
    {
        //receives an array (can use the individual photo data returned
        //from an API call) and returns a URL (doesn't mean that the
        //file size exists)
        $sizes = array(
                "square" => "_s",
                "thumbnail" => "_t",
                "small" => "_m",
                "medium" => "",
                "large" => "_b",
                "original" => "_o"
                );

        $size = strtolower($size);
        if (!array_key_exists($size, $sizes)) {
            $size = "medium";
        }

        if ($size == "original") {
            $url = "http://farm" . $photo['farm'] . ".static.flickr.com/" . $photo['server'] . "/" . $photo['id'] . "_" . $photo['originalsecret'] . "_o" . "." . $photo['originalformat'];
        } else {
            $url = "http://farm" . $photo['farm'] . ".static.flickr.com/" . $photo['server'] . "/" . $photo['id'] . "_" . $photo['secret'] . $sizes[$size] . ".jpg";
        }
        return $url;
    }