Cart.php

<?php

class Cart {
  
  private $status;
  
  public function __construct() {
      $this->status = new EmptyCartStatus($this);
  }
  
  public function setStatus(CartStatus $cartStatus) {
    $this->status = $cartStatus;
  }
  
  public function checkout() {
    $this->status->checkout();
  }
  
  public function whatever() {
    $this->status->whatever();
  }
  
}

CloseCartStatus.php

<?php

class CloseCartStatus implements CartStatusInterface {
    
    private $cart;

    public function __construct(Cart $cart) {
      $this->cart = $cart;
    }

    public function checkout() {
      throw new Exception('you cannot checkout an aldready close cart');
    }
    
    public function whatever() {
        //logic
    }

}

EmptyCartStatus.php

<?php

class EmptyCartStatus implements CartStatusInterface{
    
    private $cart;

    public function __construct(Cart $cart) {
      $this->cart = $cart;
    }

    public function checkout() {
      throw new Exception('you cannot checkout an empty cart');
    }
    
    public function whatever() {
        //logic
    }
}

OpenCartStatus.php

<?php

class OpenCartStatus implements CartStatusInterface{
    
    private $cart;

    public function __construct(Cart $cart) {
      $this->cart = $cart;
    }

    public function checkout() {
      //logica di checkout...
    }
    
    public function whatever() {
        //logic
    }

}