Possible Duplicate:
Headers already sent by PHP
I am new in object-oriented PHP. This code works well in function php but it makes some error in OOP PHP.
When i open index.php then error message as like as show below but all other code works well ::
Warning: session_start() [function.session-start]: Cannot send session cookie - headers 
  already sent by (output started at 
  E:\XAMPP\xampp\htdocs\photo_gallery_new\public\layout\header.php:14) in 
  E:\XAMPP\xampp\htdocs\photo_gallery_new\Includes\session.php on line 15
  Warning: session_start() [function.session-start]: Cannot send session cache limiter -
   headers already sent (output started at 
  E:\XAMPP\xampp\htdocs\photo_gallery_new\public\layout\header.php:14) in
   E:\XAMPP\xampp\htdocs\photo_gallery_new\Includes\session.php on line 15
The code ::
index.php
 <?php  include ("../public/layout/header.php"); ?> // for design   header
<h2> Menu </h2>
 <?php
require_once ("../Includes/initiate.php");   // it makes error 
//require_once ("../Includes/user.php");    // it works 
//require_once ("../Includes/database_object.php");  // it makes error      
echo $user->full_name();
echo "<hr />";
  ?>
</div>   
  <?php  include ("../public/layout/footer.php");   ?>  // design for footer
user.php
<?php
require_once ("database.php");
 class USER extends  databaseObject {  // it makes error
//class USER {  // but it works         
     public $id; public $username; public $password;    public $firstName;
         public $lastName;
     // user authenticate   
     public static function authenticate( $username="", $password="" ) { ...  } 
     public function full_name()  {..... } // find full name
     // common database action
     public static function record_all_user() { ...  } // find all user
     public static function record_by_id($id=0) {.....} // find specific user
     public static function record_by_sql($sql="") { ...... }  // check sql
    ?>
header.php
 <html>
  <head>
 <title> Photo Gallery </title>
<link href="stylesheet/main.css" media="all" rel="stylesheet" type="text/css" />
  </head>
<body>
   <div id="header">
     <h1> PHOTO GALLERY </h1>
   </div>
<div id="main">
session.php
<?php
   class Session {          
    private $logged_in = FALSE;
    public $user_id;            
    function  __construct()  {
        session_start();
        $this->check_login();           
          .............................
            }
    public function is_logged_in() {....} // check loged in  
    public function login($user) { .....  }
    public function logout() {
        unset($_SESSION['user_id']);
        unset($this->user_id);
        $this->logged_in = FALSE;
    }
    private function check_login()  {... .}  // check login
}    
$session = new Session();
 ?> 
initiate.php
 <?php
  require_once ("Config.php");
require_once ("Functions.php");
require_once ("session.php");
require_once ("Database.php");
require_once ("database_object.php");  // this makes error
require_once ("user.php");
 ?>
database_object.php
 <?php
require_once ("database.php");
   class databaseObject {
}
$newClass = new databaseObject();
  ?>    
 
     
     
     
     
     
    