PHP multi-key stable sort with nulls last

<?php
function sort_records(array $rows, array $keys): array
{
    $indexed = [];
    foreach ($rows as $i => $row) {
        $indexed[] = [$i, $row];
    }
    usort($indexed, function (array $a, array $b) use ($keys): int {
        foreach ($keys as $key => $dir) {
            $left = $a[1][$key] ?? null;
            $right = $b[1][$key] ?? null;
            if ($left === $right) continue;
            if ($left === null) return 1;
            if ($right === null) return -1;
            $cmp = is_string($left) && is_string($right) ? strcmp($left, $right) : $left <=> $right;
            return $dir === 'desc' ? -$cmp : $cmp;
        }
        return $a[0] <=> $b[0];
    });
    return array_column($indexed, 1);
}