PHP simple singleton don't work

Question

I am trying to make a singleton pattern but it seems I am making mistake somewhere. Everytime i try to create an instance from the class Cart it makes new one. Tested it with the setId() and getId() functions. It returns different number everytime.

class Cart
{
    private $cartQuantity = 0;

    private static $instance;

    private $id;

    private function __construct(){}

    public function addQuantity($quantity){
        $this->cartQuantity += $quantity;
    }

    public function getQuantity(){
        return $this->cartQuantity;
    }

    public function setId(){
        $this->id = rand(0, 10);
    }

    public function getId(){
        return $this->id;
    }

    public static function startCount(){
        if(self::$instance === null){
            self::$instance = new Cart();
            self::$instance->setId();
        }

        return self::$instance;
    }
}

$inst = Cart::startCount();
echo $inst->getId();

What am I doing wrong? Seems like legit block of code to me :/ Thank you in advance!

"What am I doing wrong" - I think, testing. https://3v4l.org/V9Y9b The code successfully returns same id. — svgrafov, Nov 30 '17 at 15:08
Hm. OK! Thank you very much! Guess the problem is somewhere else. Will keep searching for it :) — Toma Tomov, Nov 30 '17 at 15:11
Since you are making card. I'm guessing you expect the singleton to keep is data between each requests? Maybe you're looking for [session](http://php.net/manual/en/session.examples.basic.php). -- [The Singleton will only live for the current request](https://stackoverflow.com/questions/11633229/singleton-pattern-in-php-how-can-i-save-state-between-requests). — Clément Baconnier, Nov 30 '17 at 15:35

score 0 · Answer 1 · answered Nov 30 '17 at 15:20

Your singleton is working perfectly. I have tested it 10 times with the following code:

$inst = Cart::startCount();
echo $inst->getId();

$winst = Cart::startCount();
echo $inst->getId();

$ninst = Cart::startCount();
echo $ninst->getId();

$ninst->setId();
echo $ninst->getId();
echo $winst->getId();

$ginst = Cart::startCount();
echo $ginst->getId();

Results:

If you change the ID, it keeps beeing changed. Otherwise always the same is returned. This is exactly what you should expect (note that rand(0,10)) also can deliver 10.

There is never more than one instance.

PHP simple singleton don't work

1 Answers1