recursively flatten a multidimensional array

ArrayFlat.php

<?php

class ArrayFlat
{
    public function execute($arg): array
    {
        return is_array($arg) ? array_reduce($arg, function ($c, $a) {
            return array_merge($c, $this->execute($a));
        }, []) : [$arg];
    }
}

use PHPUnit\Framework\Assert;
use PHPUnit\Framework\TestCase;

class ArrayFlatTest extends TestCase
{
    /**
     * @test
     */
    public function i_should_be_able_to_flatten_a_multidimensional_array()
    {
        $arrayFlat = new ArrayFlat();

        $flat = $arrayFlat->execute([[1, 2, [3]], 4]);
        Assert::assertEquals([1, 2, 3, 4], $flat);

        $flat = $arrayFlat->execute([1, 2, [3, 4, 5, 6], 7, 8, [9, 10, [11, 12]]]);
        Assert::assertEquals([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], $flat);
    }
}