different explanation - php

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 ;-) )