在两个不同的类中引用相同的变量

Question

<?php

class Foo{
    public $basket;

    public function __construct()
    {
        $this->basket = 1;
    }

    public function getBasket(){
        return $this->basket;
    }
}

class Bar{
    public function __construct(&$basket)
    {
        $basket++;
    }
}

$newFoo = new Foo();
$newBar = new Bar($newFoo->getBasket());

echo $newFoo->getBasket();

?>

I am hoping to initialise the $basket value in one class and manipulate the same variable via another class. Unfortunately, I keep getting the "Notice: Only variables should be passed by reference in " error message.

Question: How can I change the code to make this happen? Thank you.

Answer 1

Change

$newBar = new Bar($newFoo->getBasket());

To

$basket = $newFoo->getBasket();
$newBar = new Bar($basket);

The first way doesn't work because PHP doesn't have any variable with which to hold the value you're passing to new Bar() As a consequence, nothing can be passed by reference.

The second way works because the $basket var is a fixed reference in memory, so it can be passed by reference to new Bar()

You asked in comments:

have changed my code to yours. echo $newFoo->getBasket(); produces 1 (I was hoping for 2).

1 is produced because each call to getBasket() gives you a fresh copy of the class variable. The $basket that I passed to new Bar() equals 2, but that's not what you're echoing.

If you want the result of getBasket() and the variable $basket to refer to the same reference in memory, you need to make two changes:

1 Change the function declaration to:

public function &getBasket()

2 Change how you store the function result to:

$basket = &$newFoo->getBasket();

Now your echo will return 2 because you would have a unique basket reference throughout your code

See the docs