First, let's define what types of collections there are:

A collection of elements associated with a numeric index. The index always starts at 0.
A collection of elements associated with a key value. The key values can be of any type.
A collection of unique elements. A set only accepts elements if it doesn't contain it already.


use Elephox\Collection\ArrayList;

$list = ArrayList::from([5, 1, 2, 6, 3, 2]);

echo $list[0]; // echoes '5'
echo $list[5]; // echoes '2'

$list[] = 4; // $list->toArray() == [5, 1, 2, 6, 3, 2, 4]

echo count($list); // echoes '7'

An ArrayList implements ArrayAccess, Countable and IteratorAggregate, meaning you can use it as a normal array (accessing elements by index, using it in a foreach, counting using count() and checking for indices using array_key_exists).

It is useful for a lot of scenarios, where you would normally use a native PHP array. ArrayLists however have some significant advantages over a normal array: they implement KeyedEnumerable.

With KeyedEnumerables, you have a lot of useful functions for sorting, filtering and mapping than are available with sort(), array_filter(), array_map(), etc. You are also more flexible, because it is an object and not a scalar value, meaning you can expand the normal functionality with your business logic.

Currently ArrayList is the only class implementing GenericList. You are free to implement it yourself and to help you, Elephox provides a trait with only one abstract method for you to implement: IsKeyedEnumerable.


use Elephox\Collection\ArrayMap;

$map = ArrayMap::from(['a' => 1, 'b' => 3, 'c' => 6, 'd' => 2]);

echo $map['a']; // echoes '1'

$map['e'] = 4; // $map->toArray() == ['a' => 1, 'b' => 3, 'c' => 6, 'd' => 2, 'e' => 4]
$map->put('e', 3); // same effect as above, updates the value of $map['e'] to be 3 

echo count($map); // echoes '5'

Maps are used to associate keys with values. For an ArrayMap, keys are restricted to normal PHP array keys (int or string). In case you need to map objects to values (like SplObjectStorage), you can use ObjectMaps:

use Elephox\Collection\ObjectMap;

$objA = new \stdClass();
$objA->title = 'Object A';

$objB = new \stdClass();
$objB->title = 'Object B';

$map = new ObjectMap();
$map->put($objA, ['status' => 'ok']);
$map->put($objB, ['status' => 'failed']);

foreach ($map as $key => $value) {
    echo $key->title . ' has status ' . $value['status'];

// prints:
// Object A has status ok
// Object B has status failed

An ObjectMap has the advantage of being able to use rich objects as keys. The values can be objects too, without having to keep two arrays in sync to store both objects.

As you might have guessed: ArrayMap and ObjectMap also implement KeyedEnumerable! This gives you plenty of functions to achieve almost everything you want.


use Elephox\Collection\ArraySet;

$set = new ArraySet();

$set->add('hello'); // returns true, because the value wasn't part of the set
$set->add('dear');  // also returns true
$set->add('world'); // also returns true
echo count($set); // echoes '3'

$set->add('hello'); // returns false, because the element was already part of the set
echo count($set); // echoes '3'

$set->remove('hello'); // returns true, because it was part of the set
echo count($set); // echoes '2'

$set->remove('hello'); // returns false, because the element didn't exist in the set to begin with
echo count($set); // echoes '2'

// remove everything that contains 'o' from the set
$set->removeBy(fn ($v) => str_contains($v, 'o')); // returns true, since at least one element was removed
echo count($set); // echoes '1' (only 'dear' remains in the set)

Sets are a great way to keep track of a list of unique elements.

Sets allow you to specify a custom compare function, which determines if two elements should be considered equal:

use Elephox\Collection\ArraySet;

$a = new \stdClass();
$a->weight = 0;

$b = new \stdClass();
$b->weight = 1;

$c = new \stdClass();
$c->weight = 2;

$d = new \stdClass();
$d->weight = 1;

$uniqueWeightsSet = new ArraySet(comparer: fn ($a, $b) => $a->weight === $b->weight);
$uniqueWeightsSet->add($a); // true
$uniqueWeightsSet->add($b); // true
$uniqueWeightsSet->add($c); // true
$uniqueWeightsSet->add($d); // false, $d->weight === $a->weight, so the element is considered to be a part of the set already 

Enumerable & KeyedEnumerable

Enumerables in Elephox were inspired and heavily influenced by the C# IEnumerable. They provide a lot of functionality by chaining and combining iterators.

Nice to know

An iterator is an object which is used by many languages (including PHP) to loop over a collection using while-loops. This reduces overhead when implementing other loops such as for and foreach since only while needs to be implemented and the others are inferred with syntactic sugar.
A PHP iterator only has a small set of functions:
  • current(): returns the current value of the iterator
  • key(): returns the current key of the iterator
  • next(): instructs the iterator to move to the next element
  • valid(): used to check if the iterator has reached the end of the collection
  • rewind(): reset the iterator to the beginning

First, let's look at how a foreach-loop loops over an array:

$array = [1, 2, 3];
foreach ($array as $index => $value) {
    echo $index . " -> " . $value . "\n";

// prints:
// 0 -> 1
// 1 -> 2
// 2 -> 3

Now, let's look at what goes on behind the scenes:

$array = [1, 2, 3];

$iterator = new \ArrayIterator($array);
while ($iterator->valid()) {
    echo $iterator->key() . " -> " . $iterator->current() . "\n";


// prints:
// 0 -> 1
// 1 -> 2
// 2 -> 3

PHP can use an ArrayIterator and a while-loop to implement a foreach-loop! In fact, a foreach-loop can use any object implementing Traversable (which is a parent of Iterator).

You cannot directly implement Traversable though. So you need to implement a subtype (Iterator or IteratorAggregate) to pass the object into a foreach-loop.

Enumerables in Elephox all implement the IteratorAggregate, meaning they have a method - getIterator() -, which returns an Iterator for the given enumerable. The Enumerable and KeyedEnumerable interfaces use said chaining and combining of iterators to efficiently implement a lot of useful functions.

Bonus fact

You can also represent a foreach-loop using a for-loop and iterators:
$array = [1, 2, 3];

$iterator = new \ArrayIterator($array);
for ($iterator->rewind(); $iterator->valid(); $iterator->next()) {
    echo $iterator->key() . " -> " . $iterator->current() . "\n";

// prints:
// 0 -> 1
// 1 -> 2
// 2 -> 3
Do you recognize the default $i operations being replaced by iterator calls? Pretty neat, huh?

The (key-)difference

Elephox differentiates between collections having keys (maps and lists) and collections being keyless (sets).

In keyed collections, the key/index decides whether to add, update or remove an element, whereas in keyless collections, only the value is important.


The traits IsEnumerable and IsKeyedEnumerable help you to implement GenericEnumerable and GenericKeyedEnumerable yourself. Both traits have only one abstract method you need to implement: getIterator(): Iterator. Every other method is implemented using the iterator returned from this method.


use Elephox\Collection\Contract\GenericEnumerable;
use Elephox\Collection\IsEnumerable;
use ArrayIterator;

class MyArrayEnumerable implements GenericEnumerable {
    use IsEnumerable;

    public function __construct(private array $elements) {}

    public function getIterator() : ArrayIterator
        return new ArrayIterator($this->elements);