public static function getAdmin($username)
 {
     $db = Connection::getInstance();
     $mysqli = $db->getConnection();
     $query = "select * from admin_user where username='******'";
     $result = $mysqli->query($query);
     $user = new Admin_user();
     if ($result->num_rows > 0) {
         if ($row = $result->fetch_assoc()) {
             $user->setUsername($row["username"]);
             $user->setPassword($row["password"]);
         }
     }
     return $user;
 }