create a list which alternates between elements of the two input lists, starting with the first element of the first list, then the first element of the second list and so on, until one of the lists doesn't have the next element, then you stop.

UnevenListsZip.php

<?php

function zipLists(array $first, array $second, $index = 0, $carry = []): array
{
    $firstValue = $first[$index] ?? null;
    if(is_null($firstValue)) {
        return $carry;
    }
    $carry[] = $firstValue;


    $secondValue = $second[$index] ?? null;
    if(is_null($secondValue)) {
        return $carry;
    }
    $carry[] = $secondValue;

    $index = $index + 1;

    return zipLists($first, $second, $index, $carry);
}

zipLists([], []); // []
zipLists([], [1,2,3]); // []
zipLists([1,2,3], []); // [1]
zipLists([1,2], [3]); // [1,3,2]
zipLists([1,2,3,4,5], [11, 12, 13]); // [1,11,2,12,3,13,4]
zipLists([9,9,9], [8,8,8]); //[9,8,9,8,9,8]
zipLists([1,2,3], [4,5,6,7]); //[1,4,2,5,3,6]