<?php

$start = microtime(true);

function permutation($warehouses)
{
    if (array_search(null, $warehouses) !== false) {
        for ($i = 0; $i < count($warehouses); $i++) {
            if (!$warehouses[$i]) {
                $warehouses[$i] = count(array_filter($warehouses)) + 1;
                permutation($warehouses);
                $warehouses[$i] = null;
            }
        }
    } else {
//        foreach ($warehouses as $warehouse) {
//            if ($warehouse) {
//                echo $warehouse . ' ';
//            }
//        }
//        echo PHP_EOL;
    }
}

$warehouses = array_fill(0, 9, null);

permutation($warehouses);

echo (microtime(true) - $start) / 60 . PHP_EOL;
echo memory_get_peak_usage(true) / 1014 . PHP_EOL;