Strange operator

[Plookie]

It's been a while since I've developed in PHP and maybe I've forgotten too much, but I haven't been able to find anything in the PHP docs about this and it has me stumped. I'm sure it'll be an easy one for the experts here, tho.

I inherited some code from a developer that is no longer here or accessible. One part of the code gets an instance of a singleton object using the following:

$o =& App_Object::getInstance();

I've never come across the "=&" operator. Outside of a simple assignment, I can't figure out what he was trying to do.

And, of course, the code compiles and works fine.

Have I just been staring at this for too long? :o

Thanks in advance,

Paul

[jkurrle]

One way to access the data outside the function, without using an argument is by using a reference to a variable as an argument. You can do this by placing an ampersand “&” in front of the argument, when you define the function. This way, the function will directly access the variable through the reference, and the variable can be assigned and read at the same time.

$exchange_rate = 1.27;

function add_tax_and_calculate_value_from_foreign($price,$items, &$rate) //notice the ampersand “&”, telling PHP to treat the variable “$rate” as a reference
{
$rate .= 0.01; //adds a 0.01 tax
return ($price * $items * $exchange_rate);
}

add_tax_and_calculate_value_from_foreign(15, 4, $exchange_rate)
print $exchange_rate; //the new exchange rate is 1.28, because the function has incremented it

Credit: http://www.softwareprojects.org/php-functions-12.htm

[Plookie]

Wow, I'm hanging my head in shame after that one. I had completely overlooked reference assignments.

Thanks for the quick reply!

[Weedpacket]

Of course, this was only necessary in PHP 4, which would return a copy of the object instead of a reference as is done in PHP 5.

[jkurrle]

Ok, I got totally lost on that one. Could you rephrase that, for my poor PHP4 entrenched mind and provide a PHP5 example to contrast with?

[laserlight]

Ok, I got totally lost on that one. Could you rephrase that, for my poor PHP4 entrenched mind and provide a PHP5 example to contrast with?

Well, let's start with an example in PHP 4. Suppose we have:

<?php
class X {
    var $x;

function X($x) {
    $this->x = $x;
}

function get() {
    return $this;
}
}

$a = new X(1);
echo $a->x . ' ';

$b = $a->get();
echo $b->x . ' ';

$b->x = 2;

echo $a->x . ' ' . $b->x;
?>

We will find that '1 1 1 2' is printed. The reason is that $b->x = 2 only sets the $x member in $b, not $a. But if we switch to PHP 5 and run the same code, we will get '1 1 2 2'. The reason is that $a->get() returns a reference to the $this object, i.e., $b now points to the same object as $a.

However, the unfortunate part is that this is like Java's reference mechanism, not C++, whereas the usual PHP pass by reference mechanism is like C++, not Java. In PHP 5 $a->get() returns a reference that acts like a pointer (but without pointer syntax). So if we were to change $b instead of changing $b->x:

$b = new X(2);

... we will find that the output is back to '1 1 1 2'. The reason is that $b = new X(2) makes $b point to the new X object, without making $a do the same. $a still points to the first X object with the $x member having a value of 1.

If we use the PHP 4 version:

<?php
class X {
    var $x;

function X($x) {
    $this->x = $x;
}

function& get() {
    return $this;
}
}

$a = new X(1);
echo $a->x . ' ';

$b =& $a->get();
echo $b->x . ' ';

$b = new X(2);

echo $a->x . ' ' . $b->x;
?>

Now we find that even with the use of $b = new X(2), $a->x is now 2, not 1 (i.e., we get '1 1 2 2' as the output). The reason is that $b =& $a->get() makes $b into an alias (i.e., in C++ reference style) of $a. So changing $b itself changes $a.

[Weedpacket]

In PHP 5, if $a=$b and $b is an object, then $a and $b will both be pointing to the same object. Compare this in PHP4 and PHP5:

$b = new stdClass;
$b->foo = 'FOO!';
$a = $b;
$a->foo = 'BAR!';
echo $b->foo;

And objects are passed to and returned from functions by reference automatically. No "&". In fact, if you do want to copy an object, PHP 5 has the [man]clone[/man] keyword.

In other words, objects in PHP5 work pretty much like objects in other languages that support them 🙂

[jkurrle]

So, is the ampersand still used in PHP5? If so, is it deprecated, or is it used in another fashion now? If I had some PHP4 code with the ampersand in it will it still run correctly? Just curious. I never used it before, but if I'm remediating someone else's code someday, it would be nice to know.

Also, how does this differentiate from global variables? Granted, you aren't supposed to use them anyway, but it seems like you can get similar effect from simply declaring a global variable, or am I totally off in left field here?

[laserlight]

So, is the ampersand still used in PHP5?

References are still available in PHP 5, and in fact you may find pass by reference to be useful. What PHP 5 makes unnecessary is return by reference and assignment by reference.

If I had some PHP4 code with the ampersand in it will it still run correctly?

As I demonstrated earlier, there is a small chance that existing PHP 4 code that returns references will behave differently from the otherwise equivalent PHP 5 code.

Also, how does this differentiate from global variables? Granted, you aren't supposed to use them anyway, but it seems like you can get similar effect from simply declaring a global variable, or am I totally off in left field here?

Pass by reference is more flexible than global variables since the (formal) parameter name can be different from the (actual) argument name. It is also more explicit since you have to pass the argument instead of the function declaring the global variable by itself, behind the scenes.

[bpat1434]

The ampersand is not used in PHP 5 as a "pass by reference" operator. If you do happen to use it, PHP will let you know 😉 Now, that's not to say the code would fail. I believe that it just raises an E_WARNING level alert, or maybe even E_STRICT; however, your code should still "work". If it doesn't, you know you need to get into it and start correcting things. If you do need a direct copy of an object in PHP 5, you can use the [man]clone[/man] keyword.

[Weedpacket]

laserlight wrote:

Pass by reference is more flexible than global variables since the (formal) parameter name can be different from the (actual) argument name. It is also more explicit since you have to pass the argument instead of the function declaring the global variable by itself, behind the scenes.

And if you have more than one instance of a given function being executed at a particular time (because div() called ul() called li() called div()....), each instance can have different arguments; whereas they'd all have to share the same global variable.