Re-cycle an array as needed to populate a new column in a 2d array

I have a 2d array and a flat array which need to be merged in a special way.

$arr1 = [
    ['a'],
    ['b'],
    ['c'],
    ['d'],
    ['e'],
];

$arr2 = ['1', '2'];

Output should be

[
    ['a', 1],
    ['b', 2],
    ['c', 1],
    ['d', 2],
    ['e', 1],
]

How do I continuously access the elements of the second array to fill add the new column to the first array?

This version will allow $arr2 to contain any number of values, should that be a requirement:

<?php

$arr1 = [
    ['a'], ['b'], ['c'], ['d'], ['e'],
];

$arr2 = ['1', '2'];

// wrap the array in an ArrayIterator and then in an 
// InfiniteIterator - this allows you to continually
// loop over the array for as long as necessary

$iterator = new InfiniteIterator(new ArrayIterator($arr2));
$iterator->rewind(); // start at the beginning

// loop over each element by reference
// push the current value in `$arr2` into 
// each element etc.
foreach ($arr1 as &$subArray) {
    $subArray[] = $iterator->current();
    $iterator->next();
}

print_r($arr1);

This yields:

Array
(
    [0] => Array
        (
            [0] => a
            [1] => 1
        )

    [1] => Array
        (
            [0] => b
            [1] => 2
        )

    [2] => Array
        (
            [0] => c
            [1] => 1
        )

    [3] => Array
        (
            [0] => d
            [1] => 2
        )

    [4] => Array
        (
            [0] => e
            [1] => 1
        )

)

Hope this helps :)