Passing by reference

[mostafatalebi]

Hello is "passing by reference" which is  indicated like &$variable, is used to reduce memory consumption? or it has other uses?

[cpd]

No, the PHP Compiler will optimise your code. The reference is merely a means of creating another variable for the same content; a reference. Its not a pointer and "they are not actual memory addresses" - http://www.php.net/manual/en/language.references.whatare.php

 

Passing by reference allows you to directly manipulate content within a function/method of a variable defined outside the scope of the function/method.

Edited June 5, 2013 by cpd

[kicken]

It's used to allow a function the ability to modify a variable. One example is to return an error message from a function, such as how fsockopen does. Another example would be to directly modify an array, such as what sort and related functions do.

 

Passing a variable by reference won't save any memory generally speaking. PHP won't copy a variable's value until it needs (due to a write operation) so if your function never makes a write statement to the parameter then the variable never gets copied and no extra memory is used.

[mostafatalebi]

I'm confused. It seems the php document is saying the same thing as I but in other words. I say it "reduces the memory consumption". Now read the below example of mine and correct me if I'm wrong:

 

EXAMPLE:

 

we have variable $x = "this is a string"; then  I do &$s = $x; I think it means that, "this is a string" has one location in the memory, then two variables with two different names can access it(passing by reference). While:

 

$x = "this is a string"; $s = $x; makes a copy of "this is a string" as variable $x content. Thus, logically, now it occupies two locations of the memory (regular variable assignment). Am I right?

[Jessica]

No, the PHP Compiler will optimise your code. The reference is merely a means of creating another variable for the same content; a reference. Its not a pointer and "they are not actual memory addresses" - http://www.php.net/manual/en/language.references.whatare.php

@mostafatalebi - Did you read this? Edited June 5, 2013 by Jessica

[mostafatalebi]

@mostafatalebi - Did you read this?

I have. But did you pay attention to "no" by him? So he told that my first justification is not true. This added some confusion to me, therefore I ask my question again with some points added to get a complementary answer with regard to theirs.

[Jessica]

He said No as in they are not "used to reduce memory consumption". He backed that up by showing the manual which says "The reference is merely a means of creating another variable for the same content; a reference. Its not a pointer and "they are not actual memory addresses"

 

Your reply talked about memory addresses. Which it specifically says a reference IS NOT.

 

You said "correct me if I'm wrong:" - You're wrong.

[cpd]

...is used to reduce memory consumption?

 

 

No.

 

Arguably I should have been clearer as to which question I was answering. 

 

Thanks for clarifying Jessica.

Edited June 5, 2013 by cpd

[mostafatalebi]

then please explain this:

according to this explanation: "The reference is merely a means of creating another variable for the same content":

what is the difference between regular assignment ($x = $s) and passing by reference?

 

 

Thanks in advance

[cpd]

Function/methods within PHP replicate variables passed to them via parameters therefore, you are manipulating a replica of the variable not the variable itself. When passing by reference, you are then manipulating the actual value itself even though its defined outside the scope of the method/function.

 

E.g.

function plusTwo($var) {
    $var+= 2;
}

$a = 1;
plusTwo($a);
echo $a; // Output is 1 still because $var is a replica of the value passed in.

/* ------------- */

function plusTwo(&$var) {
    $var+= 2;
}

$a = 1;
plusTwo($a);
echo $a; // Output is 3 because you passed the variable in by reference and the value was directly manipulated

With regards to assignments: 

$a = 2;
$b = 4;

$c =& $a;
$d = $b;

echo $a; // Output is 2
echo $b; // Output is 4

echo $c; // Output is 2
echo $d; // Output is 4

$c+= 1;
$d+= 2;

echo $a; // Output is 3 because $c directly manipulated the value it references to
echo $b; // Output is still 4

echo $c; // Output is 3
echo $d; // Output is 6 because $d is a replica of $b and therefore has a different value

Edited June 5, 2013 by cpd

[mostafatalebi]

oh! very comprehensive definition the last one. thanks a lot. great. I needed such explanation. I was totally in another page when thinking about references. Thanks buddy

[kicken]

$x = "this is a string"; $s = $x; makes a copy of "this is a string" as variable $x content. Thus, logically, now it occupies two locations of the memory (regular variable assignment). Am I right?

$s = $x will make what is known as a copy-on-write reference to $x. In other words both $x and $s point to the same value, just like if you had use a reference. The difference is that if you ever do anything that would cause either of them to change, eg $s=strtolower($s); then PHP will copy them at that point.

 

So until you perform some sort of modification to either variable, they still only occupy a single instance of memory and there is no additional overhead copying values over. As such, using a reference merely to save memory/reduce overhead is unnecessary. Only use a reference if you have another reason for doing so.

Edited June 5, 2013 by kicken