PHP array copy semantics: what it does when members are references, and where is it documented?

This code

<?php

$a = 10;
$arr1 = array(&$a);

$arr1[0] = 20;
echo $a; echo "\n";

$arr2 = $arr1;
$arr2[0] = 30;

echo $a;

produces

20
30

Obviously reference array members are "preserved", which can lead, for example, to some interesting/strange behavior, like

<?php

function f($arr) {
    $arr[0] = 20;
}

$val = 10;
$a = array(&$val);

f($a);

echo $a[0];

?>

outputting

20

My question is: what is it for, where is it documen开发者_如何学Cted (except for a user comment at http://www.php.net/manual/en/language.types.array.php#50036) and the Zend Engine source code itself?

PHP's assignment by reference behavior is documented on the manual page "PHP: What References Do". You'll find a paragraph on array value references there, too, starting with:

While not being strictly an assignment by reference, expressions created with the language construct array() can also behave as such by prefixing & to the array element to add.

The page also explains your why your first code behaves like it does:

Note, however, that references inside arrays are potentially dangerous. Doing a normal (not by reference) assignment with a reference on the right side does not turn the left side into a reference, but references inside arrays are preserved in these normal assignments. This also applies to function calls where the array is passed by value.