permutation_test.php

<?php

function test_permutations_rob($items, $perms = [], &$return = []) {
    if (empty($items)) {
        $return[] = $perms;
    } else {
        sort($items);
        $prev = false;
        for ($i = count($items) - 1; $i >= 0; --$i) {
            $newitems = $items;
            $tmp = array_splice($newitems, $i, 1)[0];
            if ($tmp != $prev) {
                $prev = $tmp;
                $newperms = $perms;
                array_unshift($newperms, $tmp);
                test_permutations_rob($newitems, $newperms, $return);
            }
        }
        return $return;
    }
}

function test_permutations_yitsushi(array $stack) {
    if (count($stack) < 2) {
        yield $stack;
    } else {
        foreach (test_permutations_yitsushi(array_slice($stack, 1)) as $iteration) {
            foreach (range(0, count($stack) - 1) as $i) {
                yield array_merge(
                    array_slice($iteration, 0, $i),
                    [$stack[0]],
                    array_slice($iteration, $i)
                );
            }
        }
    }
}

function executePermutationWithTimer(callable $function) {
    $base_array = [1, 2, 3, 4, 5, 6, 7, 8];

    $start = microtime(true);
    foreach($function($base_array) as $item) {}
    $end = microtime(true);

    printf("[%30s] Total Execution Time: %f\n", $function, ($end - $start));
}

$functions = array_filter(
    get_defined_functions()['user'],
    function($item) {
        return substr_compare($item, 'test_', 0, 4) == 0;
    }
);

foreach($functions as $f) {
    executePermutationWithTimer($f);
}

[         test_permutations_rob] Total Execution Time: 0.328834
[    test_permutations_yitsushi] Total Execution Time: 0.079049