Snaha o pochopení OOP

class Client implements IClient{
  private $clientService;
  private $id;
  private $surname;
  private $firstName;
  atd...
  
  
  public function __construct($id = null, $surname = null, $firstName = null,
                              $cellPhone = null, $phone = null, $born = null){    
    $this->surname  = $surname;
    $this->firstName = $firstName;
    atd...
    if($id){
      $this->clientService = new ClientService($id);   
      $values = $clientService->getClientValues();
      $surname = $values['surname'];
      atd..   
    }
    else{           
      $this->clientService = new ClientService(null, $this->getValues());
      $this->id = $this->clientService->getClientId();            
    }  
  }
  
  public function getSurname(){
      return $this->surname();
  }    
  
  public function setSurname($surname){
      $this->surname = $surname;
  }
  
  atd..(další setry a getry pro ostatní proměnné)
  
  public function saveChanges(){
      $this->clientService->saveChanges();
  }
  
  public function getValues(){
      $values = array('surname' => $this->surname, 'firstName' => $this->firstName,
      atd..);
      
      return $values;
  }
}

//třída starající se o práci s databází a klientem
class ClientService{
  
  private $db;
  private $clientId;
  
  public function __construct(IClient $client){
     //zde připojení k db
     if(is_object($client)){
       $clientId = $client->getId(); 
       if(is_numeric($clientId) && $clientId>0){
        $this->id=$clientId;
       } 
       else{
        if($this->createClient($client->getValues())){
          $this->clientId = mysqli_insert_id($db);        
        }
       }
     }
     else{
      throw new Exception('Chyba!');
     }
  }
  
  //funkce která vytvoří klienta v DB
  private function createClient(IClient $client){
    $values = $client->getValues();
    $query = 'INSERT INTO .....'; //samozřejmě bych to escapoval apod. jde pouze o nástin
    return mysql_query($query);
  }
  
  public function saveChanges(IClient $client){
    $values = $client->getValues();
    $query = 'UPDATE ......';
    return mysql_query($query);
  } 
  
  public function getClientId(){
    return $this->idClient;
  }   
}

interface IClient{
  public function getValues();
}

class Visit extends Client implements IVisit{
  private $visitService;
  private $id;
  private $date;
  private $info;
  atd...
  
  public function __construct($clientId, $idVisit){
    parent::__construct($clientId);
    if($idVisit){
      $visitService = new VisitService($this);
      $values = $visitService->getVisitValues();
      $this->id = $values['id'];
      atd...      
    }
    else{
      $this->visitService = new VisitService($this);
      $this->id = $this->clientService->getClientId();                       
    }          
  }
  
  public function getId(){
    return $this->id;    
  }
  
  atd...
  
  public function saveChanges(){
    $this->visitService->saveChanges($this);
  }
}

interface IVisit{
  public function getValues();  
}

//třída starající se o práci s databází a třídou visit
class VisitService{
  private $visitId;
  private $db;
  
  public function __construct(IVisit $visit){
    $this->db = mysql_connect();
    if(is_object($visitId) && $visitId>0){
      $visitId = $visit->getId();
      if($visitId){
        $this->visitId=$visitId;
      }
      else{
        if($this->createVisit($visit->getValues()){
          $this->visitId = mysql_insert_id($this->db);
        }        
      }    
    }
    else{
      throw new Exception('Chyba');
    }
  }
  
  private function createVisit(IVisit $visit){
    $values = $visit->getValues();
    $query = mysqli_query('INSERT INTO ....');
    return mysqli_query($query);
  }
  
  public function saveChanges(IVisit $visit){
    $query = 'UPDATE ......';
    return mysql_query($query);
  }
  
  public function getVisitId(){
    return $this->visitId;
  }
}