private static $config;
    private $config;

    public static function connection($configure){
        self::$config = $configure;
        return  self::$config;
    }

• The Model represents your data structures. Typically your model classes will contain functions that help you retrieve, insert, and update information in your database.
• The View is the information that is being presented to a user. A View will normally be a web page, but in CodeIgniter, a view can also be a page fragment like a header or footer. It can also be an RSS page, or any other type of "page".
• The Controller serves as an intermediary between the Model, the View, and any other resources needed to process the HTTP request and generate a web page.

$config['database']['default'] = array(
 'user' => 'username',
 'password' => '',
 'host' => '127.0.0.1',
 'database' => 'test_db',
 'type' => 'mysql'

);

Имеем такой класс

Class Connection{
  private static $connection = array();
  private static $config = array();


  final public static function setupConfig(array $config){
     if(empty(self::$config) 
        self::$config = $config;
     else
        throw new Exception('Конфиг уже подключен');

  }

  final public function getConnection($server = 'default'){

    if(isset(self::$connection[$server]){
        return self::$connection[$server];

    }

    if(empty(self::$config){
       throw new Exception('Конфиг еще не подключен');
    }
    if(!isset(self::$config[$server]){

     throw new Exception('Конфигурация для '.$server.' не найдена');
    }

    $config = self::$config[$server];
    self::$connection[$server] =  new PDO($config['type'].':host='.$config['host'].';dbname='.$config['database'], $config['user'], $config['password']);
    self::$connection[$server]->exec("SET CHARACTER SET utf8");
   return self::$connection[$server];
  }


}

class DbModel{

   protected $_connection = 'default';

   final public function getConnection(){
   return Connection::getConnection($this->_connection);

   }

   public statuc function instance(){
    return new self();

   }

}

Class User extends DbModel{

   const TABLE ='tbl_user';

   public function findByUsername($username){
      
      //Экранируем
       $this->getConnection()->quote($username, PDO::PARAM_STR);

       $stmt = $this->getConnection()->prepare("SELECT * FROM ' .self::TABLE. ' where name = '?'");
       $user = $stmt->execute(array($username))->fetch(PDO::FETCH_ASSOC);
       return $user;
   }


}

if($formValid){
   $user = User::instance()->findByUsername($_POST['username']);
   if($user){
     //Ура найден пользователь

    if(md5($_POST['password']) === $user['password']){
     Ура мы авторизовались

    }

   }


}