slightly optimized version of phpnet at jesseschalken dot com's excellent array_is_list:

<?php

if (!function_exists("array_is_list")) {
    function array_is_list(array $array): bool
    {
        $i = -1;
        foreach ($array as $k => $v) {
            ++$i;
            if ($k !== $i) {
                return false;
            }
        }
        return true;
    }
}

?>

benchmarks: https://3v4l.org/9BPqL

why is this faster you ask? because post-increment does more,

here is what pre-increment actually means:
step 1: increment the value by 1.
step 2: return the value.

here is what post-increment actually means:
step 1: create a copy of the original value.
step 2: increment the original value by 1.
step 3: return the copy.

another question might be "why didn't you write `if ($k !== ++$i) {` ?  ... that is a good question! turns out that ++$i;if($k!==$i){...} is faster on PHP7 than if($k !== ++$i){...} for reasons unknown to me.. (if you have an answer, feel free to email me about it!)
 
(i have NOT checked if PHP8/jit auto-optimize this stuff, but at least back in PHP7 it's true that pre-increment is faster than post-increment, and this polyfill is primarily for PHP7)