Question

With the use of static variables and the singleton pattern, I thought that it would be easy enough to create a simple shopping cart that remembered what items where in the cart when another page was loaded.

I am having a problem of the shopping cart not remembering what was already in it when the page is refreshed.

Is there a problem with my code below or should I just use globals or a mysql database.

What is the best way to go about storing state..

<?php
//create a singleton class
class shoppingCart {

    private static $_shoppingCartItems = array();
    private static $_instance = null;

    private function __construct(){

    }

    public static function getInstance(){
        if(self::$_instance == null)
            self::$_instance = new shoppingCart();
        return self::$_instance;            
    }


    public function add(ShoppingItem $item){
        $this->_shoppingCartItems[] = $item;
    }

    public function cartCount(){                 
        return count($this->_shoppingCartItems);
    }  
}
?>

Implementation

$item = new shoppingItem();

$shoppingCart = shoppingCart::getInstance();
$shoppingCart->add($item);
$shoppingCart->add($item);

//should increment by 2 on each page load but it doesn't
echo $shoppingCart->cartCount(); 

Answer 1

I think I can see your thought pattern there but what you’re trying to do is wrong in many ways.

1. Singleton is NOT a pattern, it’s an antipattern

The Singleton is an anti-pattern and should be avoided at all costs. See this great answer by Gordon for the why.

2. HTTP is a stateless protocol.

Nothing you do in PHP alone will help you to preserve state across two requests. Your $shoppingCart is created from the scratch for each request, in fact, your whole application is. You should NOT try to persist data in objects instead you should recreate state after every request, by fetching the respective data from somewhere else. In your example probably from some sort of database nosql or sql.

3. Sessions

You can persist user specific data in the superglobal $_SESSION, but in most cases I advice against it. Your user session should hold authentication and user data but you should avoid storing all kinds data relevant for your business logic in there.