Passing parameters by reference using func_get_arg(s)

July 27, 2008 by Christoff Truter PHP  

I wanted to write a blog about func_get_arg(s) for quite a while now. Since its a very handy function in PHP, but never got around doing it.

Until I ran into an interesting issue last week, while working on a new article for my website.

Lets have a look at the function before continuing, so that everyone knows what I am talking about. What we've got here is functionality which allows developers to pass parameters to a function, without needing to literally define them.

function demonstrate()
{
    $num_args = func_num_args();
    for ($i = 0; $i < $num_args; $i++)
    {
        echo func_get_arg($i);
    }
}

demonstrate("a","b","c"); // outputs abc

The issue comes in when we want to pass parameters by reference, observe the following code.

function increment($a)
{
    $a++;
}
$value = 10;
increment(&$value);

echo $value."<br/>"; // Value correctly echoed as 11

function increment2()
{
    $a = func_get_arg(0);		
    $a++;
}

increment2(&$value);

echo $value."<br/>"; // Value still 11

At first glance one would think that this is a bug, and as a matter of fact has been reported to be one:

http://bugs.php.net/bug.php?id=6427
http://bugs.php.net/bug.php?id=14705
http://bugs.php.net/bug.php?id=15098
http://bugs.php.net/bug.php?id=16464

In the PHP documentation, it does however state that it only returns a copy of the parameters - which isnt entirely true in concept anymore. Mainly because PHP 5 introduced a new object model, objects are passed by reference (or by handle), non-objects are copied.

So even if we are to "copy" an object it will simply be a copy of its handle, meaning it will still point to the orignal object. One would need to clone it if you need a copy.

In context of the func_get_arg(s) function, objects will automatically be by reference. As for non-objects we will need to "box" them if we want to pass it by reference using this function.

$value = 11;

function increment3()
{
    $a = func_get_arg(0);		
    $a->scalar++;
}

$value = (object)$value; // Similar to Boxing
increment3($value);
$value = $value->scalar; // Essentially our unboxing
echo $value."<br/>"; // Value correctly echoed as 12

Which is a bit of a tedious (but interesting) solution as pointed out by one of my vistors (Chris) from www.pulseforce.com

The following solution he suggested makes a LOT more sense (you can read his comments further down):

<?php 

$a = 11;

function increment4()
{
	$a = func_get_arg(0);
	$a++;
	return array($a);
}

list($a) = increment4($a);

echo $a; // Value correctly echoed as 12

?>

There is however even a better solution (also pointed out by one of the visitors), by using the debug_backtrace function, we're able to pass values by reference.

$value = 11;

function increment5()
{
	$stack = debug_backtrace();
	$stack[0]["args"][0]++;
}

increment5(&$value);
 
echo $value."<br/>"; // Value correctly echoed as 12