Reference assignment operator in PHP, =&

It's two different operators. = is assignment as you probably know. And & means the variable should be accessed by reference rather than by value.

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.

$x = &$y['z'];

also has the effect of creating $y['z'] if it doesn't exist, and setting it to null.

This prevents error messages that you might have wanted to read. I haven't found documentation on this yet; possibly new in 5.3, for all I know.