Why is https://wiki.php.net/rfc/array_key_first_last_index necessary?

array_key_polyfill.php

<?php

function array_key_first(array $arr)
{
    reset($arr);
    return key($arr);
}

function array_key_last(array $arr)
{
    end($arr);
    return key($arr);
}

reset() modifies the array, forcing a copy to be made. For large arrays, this can be slow and consume memory. A significantly more performant (in PHP 7 only) polyfill for array_key_first would be:

function array_key_first(array $arr)
{
     foreach($arr as $k => $v) {
        return $k
    }

    return null;
}

array_key_last() would still suffer from all the issues presented in the RFC.

maybe

function array_key_first(array $xss)
{
    return [] === $xss
        ? null
        : array_keys(array_slice($xss, 0, 1, true))[0];
}

function array_key_last(array $xss)
{
    return [] === $xss
        ? null
        : array_keys(array_slice($xss, -1, 1, true))[0];
}