Collection.php

<?php

namespace App;

use ArrayAccess;
use ArrayIterator;
use Countable;
use IteratorAggregate;

class Collection implements Countable, ArrayAccess, IteratorAggregate
{
    public function __construct(protected array $items)
    {
        //
    }

    public static function make(array $items)
    {
        return new static($items);
    }

    public function count()
    {
        return count($this->items);
    }

    public function offsetExists($offset)
    {
        return isset($this->items[$offset]);
    }

    public function offsetGet($offset)
    {
        return $this->items[$offset];
    }

    public function offsetSet($offset, $value)
    {
        if (is_null($offset)) {
            $this->items[] = $value;
        } else {
            $this->items[$offset] = $value;
        }
    }

    public function offsetUnset($offset)
    {
        unset($this->items[$offset]);
    }

    public function getIterator()
    {
        return new ArrayIterator($this->items);
    }
}

CollectionTest.php

<?php

namespace Tests;

use App\Collection;
use IteratorAggregate;
use PHPUnit\Framework\TestCase;

class CollectionTest extends TestCase
{
    /** @test */
    function it_can_be_counted()
    {
        $items = ['one', 'two', 'three'];

        $this->assertCount(3, Collection::make($items));
    }

    /** @test */
    function it_mimics_an_array()
    {
        $items = ['one', 'two', 'three'];

        $collection = Collection::make($items);

        $this->assertEquals('one', $collection[0]);
        $this->assertEquals('two', $collection[1]);
        $this->assertEquals('three', $collection[2]);
    }

    /** @test */
    function it_can_be_iterated()
    {
        $items = ['one', 'two', 'three'];

        $collection = Collection::make($items);

        $this->assertInstanceOf(IteratorAggregate::class, $collection);

        foreach ($collection as $index => $item) {
            $this->assertEquals($items[$index], $item);
        }
    }
}