Warning: session_start(): Cannot send session cookie

<?php
require_once "config_class.php";
require_once "user_class.php";
require_once "news_class.php";
require_once "menu_class.php";
require_once "banner_class.php";
require_once "message_class.php";
 
abstract class Modules {
 
           protected $config;
    protected $user;
    protected $news;
    protected $menu;
    protected $banner;
    protected $message;
    protected $data;
    
    public function __construct($db) {
               session_start();
        $this->config = new Config();
        $this->user = new User($db);
        $this->news = new News($db);
        $this->menu = new Menu($db);
        $this->banner = new Banner($db);
        $this->message = new Message();
        $this->data = $this->secureData($_GET);
    }
    
    public function getContent() {
        $sr["title"] = $this->getTitle();
        $sr["meta_desc"] = $this->getDescription();
        $sr["meta_key"] = $this->getKeyWords();
        $sr["menu"] = $this->getMenu();
        $sr["auth_user"] = $this->getAuthUser();
        $sr["banners"] = $this->gerBanners();
        return $this->getReplaceTemplate($sr, "main");
    }
 
    abstract protected function getTitle();
    abstract protected function getDescription();
    abstract protected function getKeyWords();
    
    protected function getAuthUser() {
        return $this->getReplaceTemplate($sr, "form_auth");
    }
    
    protected function getBanners() {
        $banners = $this->banner->getAll();
        for($i = 0; $i < count($banners); $i++) {
            $sr["code"] = $banners[$i]["code"];
            $text .= $this->getReplaceTemplate($sr, "banner");
        }
        return $text;       
    }
    
    private function secureData($data) {
        foreach($data as $key => $value){
            if (is_array($value)) $this->secureData($value);
            else $data[$key] = htmlspecialchars($value);
        }
        return $data;       
    }
    
    protected function getTemplate($name) {
        $text = file_get_contents($this->config->dir_tmpl.$name.".tpl");
        return str_replace("%address%", $this->config->address, $text);
    }
    
    protected function getReplaceTemplate($sr, $template) {
        return $this->gerReplaceContent($sr, $this->getTemplate($template));
    }
    
    private function gerReplaceContent($sr, $content) {
        $search = array();
        $replace = array();
        $i = 0;
        foreach ($sr as $key => $value) {
            $search[$i] = $key;
            $replace[$i] = $value;
            $i++;
        }
        return str_replace($search, $replace, $content);
    }
    
    
}
?>

<?php
    mb_internal_encoding("UTF-8");
    require_once "lib/database_class.php";
    require_once "lib/frontpagecontent_class.php";
    
    $db = new DataBase();
    $view = $_GET["view"];
    switch ($view) {
        case "":
            $content = new FrontPageContent($db);
        break;
        default: exit;
    }
    
    echo $content->getContent();
    
?>