Examples:
$url = $api->get_media('UNsNhYrHG6e0oUtC8AyjCntIW1JYoBOmmwvM4oCcxZUBQ5PDFgeB9umDhrd9zOa-');
Result:
https://api.weixin.qq.com/cgi-bin/media/get?access_token=egpGMhgnhbrqOo77wkUS7HmEFp40bITkRZNJk1gCGTH8i-BiVxai9zs0CcWk223dz6LiypGprpLHBRL9upjKQLqPgtAnqUeK9qznUyDsNXg&media_id=CVS_UPz62LKIfDwc7bUWtI250x_KBLhOuYgkHr1GjVxJCP8N9rOYfgIKXSY5Wg9n