php - while making login page, I am unable to redirect to another page

There is an error while redirecting the page from login to index(i.e server error // error 500). I used redirect_to function to call function.php from login.php file and i have included header function in function.php file. unfortunately, there is server error.I tried to solve it but i could not.i have posted my all four file.

login.php

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


    if($session->is_logged_in()){
         redirect_to("index.php");
    }
    //remember to give your form's submit tag a name= "submit" attribute
    if(isset($_POST['submit'])){

    $username = trim($_POST['username']);
    $password = trim($_POST['password']);


    //check database to see if username/password exit.
    $found_user =  User::authenticate($username,$password);

    if($found_user){

        $session->login($found_user);
         redirect_to("index.php");
    }else{

        //username/password combo was not found in the database

         $message ="Username/password incorrect.";
          echo  $message;

    } 
    }
    else{//form has not been submitted

    $username = "";
    $password = ""; 
    }

    ?>

            $database->close_connection();
    }
    ?>
    
    
        
        
    ="text/css"/>
    

    
     

    
Photo Gallery
     
     

     
staff login


     


     

     














     
        
Username:
        

        

        
        
     
        
Password:
        

            
        
     
     
        

            
        
     
      
    
    
     

index.php

    
    require_once('../../includes/function.php');
    require_once('../../includes/session.php');
    if(!$session->is_logged_in()) {
         redirect_to("login.php");

    }
    ?>

    
    
        
            ="text/css"/>
    


    
     

    
Photo Gallery
     
     

     
staff login
    

    
Copyright,prayash 
    bhari

    
     

function.php

        ob_start();
      function strip_zeros_from_data($marked_string =""){
        //first remove the marked zeros

        $no_zeros = str_replace('*0','',$marked_string);
        //then remove any remaining marks
        $cleaned_string = str_replace('*','', no_zeors);
        return $cleaned_string;
      }
    function redirect_to($location = NULL){
        if ($location != NULL){
            header("Location : {$location}");
            exit;
        }


    }

    function output_message($message = ""){
        if($empty($message)){
            return "
{$message}
";
        }
        else{
            return "";
        }

    }
    function __autoload($class_name){
        $class_name = strtolower($class_name);
        $path = "../includes/{$class_name}.php";
        if(file_exists($path)){
            require_once($path);
        }else{
            die("the file {$class_name}.php could not found.");
        }


    }
    ob_end_flush();
    ?>

sesssion.php

// A class to help work with Sessions
//In our case, primarily to mange logging users in and out


//keep in mind when working with sessions that it is generally
//inadvisable to store DB-relate objects in sessions
class Session{ 
private $logged_in = false;
public $user_id;
function __construct(){

session_start();
$this->check_login();

if($this->logged_in){
    //actions to take right away if user is logged in
}else{
    //actions to take right away if user is not logged in
}
}  

public function is_logged_in(){
    return $this->logged_in; 
}



public function login($user){
    //database should find user based on username/password
if($user){
    $this->user_id = $_SESSION['user_id'] = $user -> id;
    $this->logged_in = true;

}



}

public function logout(){
unset($_SESSION['user_id']);
unset($this->user_id);
$this->logged_in = false;
}

private function check_login(){

    if(isset($_SESSION['user_id'])){
        $this->user_id = $_SESSION['user_id'];
        $this->logged_id = true;

    }else{
        unset($this->user_id);
        $this->logged_in = false;
    }
}
}


$session  = new Session()
?>

error message