different explanation

Question

The code

$global_obj = null;
class my_class
{
       var $value;
       function my_class()
       {
               global $global_obj;
               $global_obj = &$this;
       }
}
$a = new my_class;
$a->my_value = 5;
$global_obj->my_value = 10;
echo $a->my_value;

echoes 5, not 10.

"Upon first examination, it would seem that the constructor of my_class stores a reference to itself inside the $global_obj variable. Therefore, one would expect that, when we later change the value of $global_obj->my_value to 10, the corresponding value in $a would change as well. Unfortunately, the new operator does not return a reference, but a copy of the newly created object."

I still don't understand it, so can anyone please explain it differently, and help me understand?

Answer 1

Not sure why this is the way it works, but, if you remove the & in front of $this while assigning it to your global variable, it will work.

To illustrate that, the following portion of code :

$global_obj = null;
class my_class
{
   public $my_value;
   public function __construct()
   {
        global $global_obj;
        $global_obj = $this;
   }
}
$a = new my_class;
$a->my_value = 5;

$global_obj->my_value = 10;
echo $a->my_value;

Gives the following output :

10

Here are the differences with your code :

• I remove the & before $this : with PHP 5, there is no need for that, when working with objects
• I translated the code to real PHP 5 :
  • __construct for the constructor
  • use public/protected/private, and not var for properties

As a sidenote, the code you posted should have given you the following warning :

Strict standards: Creating default object from empty value

Notes :

• I'm using PHP 5.3.2
• E_ALL doesn't include E_STRICT (source)

---

EDIT after some more searching :

Going through the References Explained section of the PHP manual, and, more specifically the What References Do page, there is a warning given that says (quoting) :

If you assign a reference to a variable declared global inside a function, the reference will be visible only inside the function.
You can avoid this by using the $GLOBALS array.

And there is an example going with it.

Trying to use $GLOBALS in your code, I have this portion of code :

$global_obj = null;
class my_class
{
   public $my_value;
   public function __construct()
   {
        $GLOBALS['global_obj'] = & $this;
   }
}
$a = new my_class;
$a->my_value = 5;

$global_obj->my_value = 10;
echo $a->my_value;

And I get the following output :

10

Which seems to work ;-)

If I replace the __construct method by this :

public function __construct()
{
    global $global_obj;
    $global_obj = & $this;
}

It doesn't work...

So it seems you should not use global, here, but $GLOBALS.

The explanation given in the manual is :

Think about global $var; as a shortcut to $var =& $GLOBALS['var'];.
Thus assigning another reference to $var only changes the local variable's reference.

---

And, just so it's said : using global variables is generally not quite a good idea -- and, in this specific situation, it feels like a very bad idea...

(Now, if this question what just to understand why... Well, I can understand your curiosity ;-) )