Reference assignment operator in PHP, =&

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What does the =& (equals-ampersand) assignment operator do in PHP?

Is it deprecated?

assignment-operator operators php
5 Answers

The symbol & is used in various ways in PHP to represent operations with "references". The PHP manual has a section titled References Explained which every PHP programmer should read.

It's important to understand that references in PHP are not a data type, like a pointer, but a concept regarding how variables work. There is therefore no single meaning of & - you should not read it as "make a reference" - it just means "something reference-y is happening here".

In particular, the syntax $a =& $b, which can also be written $a = &$b, represents assignment by reference. It binds two variables together, so that they both point at the same piece of data. Think of the & as modifying the = rather than modifying the $b.

Once you've bound two variables together in this way, they are interchangeable - you can't say that "$a points to $b" or "$b points to $a":

$a =& $b;
$a = 42;
// both $a and $b will be 42
$b = 101;
// both $a and $b will be 101

You can also link more than two variables together as references, and again it doesn't matter which of the existing names you use on the right-hand side of the assignment:

$a =& $b;
$c =& $b;
$d =& $a;
$e =& $c;
// $a, $b, $c, $d, and $e now all point to the same data, interchangeably

However, if you put the same variable on the left-hand side, it breaks the existing link of that variable, and links it to something else:

$a =& $b;
// $a and $b are linked together

$a =& $c;
// $a is now linked to $c
// the value of $b doesn't change, but it is not linked to $a or $c

To "break" the link without making a new link, you can use the unset keyword:

$a =& $b;
$c =& $a;
// $a, $b, and $c are all linked together
unset($a);
// $b and $c are still linked together, but $a is independent

Some descriptions refer to =& as "creating or adding to a reference set". Perhaps it would have been better if it had been implemented as a function, like bind($a, $b) to highlight that both arguments are affected by the operation.

user_1579570

I'd like to draw some attention to the semantics and code styling of "Assigning By Reference". The OP's opening sentence hints toward a misconception:

What does the =& (equals-ampersand) assignment operator do in PHP?

First, let's review the dedicated section of the PHP Docs page for Assignment Operators. Notice how the = comes before the & and that the two symbols are separated. This is because they are NOT "combined operators". Semantically, it is "assigning" a "reference"; it is not a "reference assignment operator".

Second, look at how ALL of the "combined operators" are written lower on the docs page. The = is consistently the right-most symbol. This is a very important distinction because writing the & on the left of the = changes the meaning -- it becomes a combined operator ("bitwise and assignment operator") instead of an assignment to a reference.

PSR coding standards should be something that all PHP developers are aware of and strive to obey. Notice this rule of PSR-12 Section 6.2:

All binary arithmetic, comparison, assignment, bitwise, logical, string, and type operators MUST be preceded and followed by at least one space

By this rule, there should always be a space after the = operator -- this makes =& a violation.

Furthermore, there are other rules that state that there should not be a space between & and its variable/argument/function/etc.

When using the reference operator & before an argument, there MUST NOT be a space after it

TL;DR

When assigning a reference, always write the = with spaces on both sides and never write a space after &.

  • Bad: $a =& $b;
  • Good: $a = &$b;

Demonstrated consistently/correctly: https://riptutorial.com/php/example/11991/assign-by-reference

Not demonstrated consistently/correctly:

user_29434030
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