Hvad kan abstract, interface,final og hvad skal jeg bruge det til i oop?

  interface Biler
  {
    public function getYear( $model );
    public function getPrice( $model );
    public function getModels();
    public function getImages( $model );
    public function getAdditionalInfo( $model );
  }

  class Seat_Ibiza implements Biler
  {

    public function getYear( $model )
    {
      $db->queryItem("SELECT year FROM tCars WHERE model = '$model'");
      return $db->result->year;
    }

    public function getPrice( $model );
    {
      $db->queryItem("SELECT price FROM tCars WHERE model = '$model'");
      return $db->result->year;
    };


    public function getModels()
    {
      $sql = "SELECT model FROM tCarModels WHERE carType = 'SEAT' ORDER BY model ASC";
      $db->query($sql);
      return $db->result->result2array();
    };

    public function getImages( $model )
    {
      $sql = "SELECT filename, filecaption FROM tCarImages WHERE modelType = '$model' ORDER BY filename ASC";
      $db->query($sql);
      return $db->result->result2array();
    };
    
    public function getAdditionalInfo( $model )
    {
      $db->queryItem("SELECT additionalInfo FROM tCars WHERE model = '$model'");
      return $db->result->additionalInfo;
    };


  }

  class Peugeot_406 implements Biler
  {
      private $models = array();
      private $years = array();
      private $prices = array();
      private $images = array();
      private $info = array();
  
    public function __construct()
    {
      $this->_populateModels();
      $this->_populateYears();
      $this->_populatePrices();
      $this->_populateImages();
      $this->_populateInfo();
    }

    public function getYear( $model )
    {
         return $this->years[$model];
    };

    public function getPrice( $model );
    {
      return $this->prices[$model];
    };


    public function getModels()
    {
      return $this->models;
    };

    public function getImages( $model )
    {
      return $this->images[$model];
    };
    
    public function getAdditionalInfo( $model )
    {
      return $this->info[$model];
    };


    private function _populateModels()
    {
      $sql = "SELECT model FROM tCarModels WHERE carType = 'SEAT' ORDER BY model ASC";
      $db->query($sql);
      $this->models = $db->result->result2array();
    };

    private function _populateYears()
    {
      foreach($this->models as $model)
      {
        $db->queryItem("SELECT year FROM tCars WHERE model = '$model'");
        $this->years[$model] = $db->result->year;  
      }
    };

    private function _populatePrices()
    {
      foreach($this->models as $model)
      {
        $db->queryItem("SELECT price FROM tCars WHERE model = '$model'");
        $this->prices[$model] = $db->result->price;  
      }
    };

    private function _populateImages()
    {
      foreach($this->models as $model)
      {
        $sql = "SELECT filename, filecaption FROM tCarImages WHERE modelType = '$model' ORDER BY filename ASC";
        $db->query($sql);
        $this->images[$model] = return $db->result->result2array();
      }
    
    };
    
    private function_populateInfo()
    {
      foreach($this->models as $model)
      {
        $db->queryItem("SELECT additionalInfo FROM tCars WHERE model = '$model'");
        $this->info[$model] = $db->result->additionalInfo;  
      }      
    };


  }

  $bil = $_POST["bil"];
  
  if( !class_exists($bil) )
  { 
   header("Location: notfound.php");
   exit;
  }
  
  $bilObj = new $bil();

  $modeller = $bilObj->getModels();
  ...

  abstract class Biler
  {
      protected $db; // vores database abstraktionslag kan tilgås gennem denne
  
      protected function __construct()
      {
          $this->_startMysqlConnection();
      }
      
      private function _startMysqlConnection()
      {
          $this->db = new MysqlObj("localhost", "brugernavn", "kodeord", "database");
      }
  
    abstract public function getYear( $model );
    abstract public function getPrice( $model );
    abstract public function getModels();
    abstract public function getImages( $model );
    abstract public function getAdditionalInfo( $model );
  }

  class Seat_Ibiza implements Biler
  {

    public function getYear( $model )
    {
      $db->queryItem("SELECT year FROM tCars WHERE model = '$model'");
      return $db->result->year;
    }

        ...

  class Peugeot_406 extends Biler
  {
      private $models = array();
      private $years = array();
      private $prices = array();
      private $images = array();
      private $info = array();
  
    public function __construct()
    {
      parent::__construct();
      $this->_populateModels();
      $this->_populateYears();
      $this->_populatePrices();
      $this->_populateImages();
      $this->_populateInfo();
    }

    ...

<?php

  
  class Users
  {
      private $db;
      
      public function __construct()
      {
          $this->db = new Mysql();
      }
      
      public function login( $username, $password )
      {
          $username = $this->db->escape($username);
          $password = MD5($password);
          
          $sql = "SELECT userId FROM users WHERE username='".$username."' AND password='".$password."' LIMIT 1":
          $userId = $this->db->queryItem($sql);
      
          return $userId;
      
      }
      
        ...      
      
  }

?>

  $db = cCore::getResource("Databases.Mysql");