How do you pass objects by reference in PHP 5?

Question

In PHP 5, are you required to use the & modifier to pass by reference? For example,

class People() { }
$p = new People();
function one($a) { $a = null; }
function two(&$a) { $a = null; )

In PHP4 you needed the & modifier to maintain reference after a change had been made, but I'm confused on the topics I have read regarding PHP5's automatic use of pass-by-reference, except when explicity cloning the object.

In PHP5, is the & modifier required to pass by reference for all types of objects (variables, classes, arrays, ...)?

Answer 1

are you required to use the & modifier to pass-by-reference?

Technically/semantically, the answer is yes, even with objects. This is because there are two ways to pass/assign an object: by reference or by identifier. When a function declaration contains an &, as in:

function func(&$obj) {}

The argument will be passed by reference, no matter what. If you declare without the &

function func($obj) {}

Everything will be passed by value, with the exception of objects and resources, which will then be passed via identifier. What's an identifier? Well, you can think of it as a reference to a reference. Take the following example:

class A
{
    public $v = 1;
}

function change($obj)
{
    $obj->v = 2;
}

function makezero($obj)
{
    $obj = 0;
}

$a = new A();

change($a);

var_dump($a); 

/* 
output:

object(A)#1 (1) {
  ["v"]=>
  int(2)
}

*/

makezero($a);

var_dump($a);

/* 
output (same as before):

object(A)#1 (1) {
  ["v"]=>
  int(2)
}

*/

So why doesn't $a suddenly become an integer after passing it to makezero? It's because we only overwrote the identifier. If we had passed by reference:

function makezero(&$obj)
{
    $obj = 0;
}

makezero($a);

var_dump($a);

/* 
output:

int(0) 

*/

Now $a is an integer. So, there is a difference between passing via identifier and passing via reference.

Answer 2

You're using it wrong. The $ sign is compulsory for any variable. It should be: http://php.net/manual/en/language.references.pass.php

function foo(&$a)
{
$a=null;
}


foo($a);
To return a reference, use

 function &bar($a){
$a=5;
return $a

 }

In objects and arrays, a reference to the object is copied as the formal parameter, any equality operations on two objects is a reference exchange.

$a=new People();
$b=$a;//equivalent to &$b=&$a roughly. That is the address of $b is the same as that of $a 

function goo($obj){
//$obj=$e(below) which essentially passes a reference of $e to $obj. For a basic datatype such as string, integer, bool, this would copy the value, but since equality between objects is anyways by references, this results in $obj as a reference to $e
}
$e=new People();
goo($e);

Answer 3

Objects will pass-by-reference. Built in types will be pass-by-value (copied);

What is happening behind the scenes is that when you pass in a variable that holds an object, it's a reference to the object. So the variable itself is copied, but it still references the same object. So, essentially there are two variable, but both are pointing to the same object. Changes made to objects inside a function will persist.

In the case of the code that you have there (first you need $ even with &):

$original = new Object();

one($original); //$original unaffected
two($original); //$original will now be null

function one($a) { $a = null; } //This one has no impact on your original variable, it will still point to the object

function two(&$a) { $a = null; ) //This one will set your original variable to null, you'll lose the reference to the object.