Are PHP Associative Arrays ordered?

Question

I come from python background and the python datatype which is similar (a dictionary) is an unordered set of key value pairs.

I am wondering if PHP associative arrays are unordered? They appear to be ordered.

$test = array(
  'test' => 'test',
  'bar' => 'bar',
);

var_dump($test);    

var_dump(array_slice($test, 0, 1));

Test always comes before bar and I can slice this array as you see. So is this always guaranteed to be ordered across php versions? Is the order just the order that I have declared the array with? So something is internally pointing 'test' to place [0] in the array? I have read http://php.net/manual/en/language.types.array.php but it doesn't shed too much light on this issue. I appreciate your responses. Ty

Answer 1

PHP associative arrays (as well as numeric arrays) are ordered, and PHP supplies various functions to deal with the array key ordering like ksort(), uksort(), and krsort()

Further, PHP allows you to declare arrays with numeric keys out of order:

$a = array(3 => 'three', 1 => 'one', 2 => 'two');
print_r($a);

Array
(
    [3] => three
    [1] => one
    [2] => two
)
// Sort into numeric order
ksort($a);
print_r($a);
Array
(
    [1] => one
    [2] => two
    [3] => three
)

From the documentation:

An array in PHP is actually an ordered map. A map is a type that associates values to keys. This type is optimized for several different uses; it can be treated as an array, list (vector), hash table (an implementation of a map), dictionary, collection, stack, queue, and probably more. As array values can be other arrays, trees and multidimensional arrays are also possible.

Answer 2

The documentation states:

An array in PHP is actually an ordered map.

So yes, they are always ordered. Arrays are implemented as a hash table.

Answer 3

From the php manual:

Arrays are ordered. The order can be changed using various sorting functions. See the array functions section for more information.

I have relied on the fact that they are ordered and it has worked consistently in every project I've had.

Answer 4

The array is ordered but that does not mean the keys are sorted, it means that they are in a given order. Where the precise order is not specified, but it appears to be the order in which you introduced the key-value pairs in it.

To understand it, think what would it mean to not be ordered? Well think to a relation in a relational database. A relation is not intrinsically ordered: when you access it with a query the database, unless you provide an order clause, can return the same data in any order. Even if the data was not modified the same data can be returned in different order.

Answer 5

This is not simple question. The documentation in this case suggests non-obvious points in development, for example:

$a = [0 => 'first', 1 => 'second', 3 => 'fourth'];
$a[2] = 'third';
foreach ($a as $element) print($element . "\n");

Common sense tells us that when iterating this array, we'll see 'third' as the third element, but this is not true:

first
second
fourth
third

Because the elements are stored in the order they have been added, and 0-1-2-3 its just a key, not order. In common, documentation assumes that the elements of an array have some order, which is explicitly stated in the documentation, that means that you van acces to: first element, n-element, last element. But it is not clearly possible to operate them directly. That is, you can get the last element, add to the end (push/pop), you can get the first - shift, you can iterate over them (foreach), but you can't get the n-element without any code or place an element in a certain position without additional code. Well, the question is - how can we rely on this order, if it is required for any reason.