php - Cannot modify header information - headers already sent by (output started at. What does this mean?

I have looked at many other sites that state the problem is because of white spacing. I dont think that is the problem since I dont leave any white spacing after the end of my php script and before the begginning of my php script. Here is the full error

: Cannot modify header information - headers already sent by (output started at /admin/login.php:19) in admin/login.php on line 22

Here is my code:

               require_once("../../includes/database.php");
        require_once("../../includes/session.php");
        require_once("../../includes/user.php");

        if($session->is_logged_in())

        {
        header("Location:index.php");
        }
        if(isset($_POST['submit']))
        {
        $username = trim($_POST['username']);
        $password = trim($_POST['password']);
        //Check Databases
        $found_user = User::authenticate($username, $password);
        $found_user->id = $found_user[0];

        if($found_user)
        {
            $session->login($found_user);
            header("Location:index.php");
        }
        else
        {
            echo "Username/password combination incorrect.";
        }
            } else { //form is not submitted

        $username = "";
        $password = "";
        }
       ?>
       
           
           
           
           
Photo Gallery
           


           
Staff Login
           

           













         
                
Username:
                

                  
                
             
            
                
 Password:
                

                  
                
            
            
                

                  
                
            
           
           
           
           
           
                    {
                $database->close_connection();
        }
           ?>