Question

Is there a function to make a copy of a PHP array to another?

I have been burned a few times trying to copy PHP arrays. I want to copy an array defined inside an object to a global outside it.

Answer 1

In PHP, all variables except objects are assigned by the mechanism called copy-on-write, while objects are assigned by reference. Which means that for the arrays with scalar values simply $b = $a already will give you a copy:

$a = array();
$b = $a;
$b['foo'] = 42;
var_dump($a);

Will yield:

array(0) {
}

Whereas with objects,

$a = new StdClass();
$b = $a;
$b->foo = 42;
var_dump($a);

Yields:

object(stdClass)#1 (1) {
  ["foo"]=>
  int(42)
}

An edge case when array elements could be objects that need to be cloned as well, is explained in another answer

You could get confused by intricacies such as ArrayObject, which is an object that acts exactly like an array. Being an object however, it has reference semantics.

Edit: @AndrewLarsson raises a point in the comments below. PHP has a special feature called "references". They are somewhat similar to pointers in languages like C/C++, but not quite the same. If your array contains references, then while the array itself is passed by copy, the references will still resolve to the original target. That’s of course usually the desired behaviour, but I thought it was worth mentioning.