Python function permutations in PHP language.

example.php

<?php

require 'permutations.php';

$timeStart = microtime(true);
$memoryStart = memory_get_usage();

$i = 0;

foreach (permutations(range(1, 3), 2) as $combo) {
    echo json_encode($combo) . PHP_EOL;
    ++$i;
}

$_timeSeconds = microtime(true) - $timeStart;
$_memoryUsageMb = (memory_get_usage() - $memoryStart) / 1024 / 1024;

echo 'Total count: ' . $i . PHP_EOL;
echo 'Time seconds, s: ' . $_timeSeconds . PHP_EOL;
echo 'Memory usage, M: ' . $_memoryUsageMb . PHP_EOL;

die;

permutations.php

<?php

function py_range($start, $stop = null, $step = null)
{
    if (null === $stop) {
        $stop = $start;
        $start = 0;
    }

    if (null === $step) {
        if ($start > $stop) {
            return [];
        }
        --$stop;

        return range($start, $stop);
    } elseif (-1 === $step) {
        if ($start > $stop) {
            list($start, $stop) = [$stop, $start];
        }
        ++$start;

        return array_reverse(range($start, $stop));
    } else {
        if ($start > $stop) {
            return [];
        }

        return range($start, $stop, $step);
    }
}

/**
 * Return successive r length permutations of elements in the iterable.
 *
 * If r is not specified or is None, then r defaults to the length of the iterable and all possible full-length permutations are generated.
 *
 * Permutations are emitted in lexicographic sort order. So, if the input iterable is sorted, the permutation tuples will be produced in sorted order.
 *
 * Elements are treated as unique based on their position, not on their value. So if the input elements are unique, there will be no repeat values in each permutation.
 *
 * Example usage,
 *
 * ```php
 * $i = 0;
 * foreach (permutations(range(1, 3), 2) as $combo) {
 *     echo json_encode($combo) . "\n";
 *     ++$i;
 * }
 * echo 'Total count: ' . $i . "\n";
 * ```
 *
 * Outputs,
 *
 * ```
 * [1,2]
 * [1,3]
 * [2,1]
 * [2,3]
 * [3,1]
 * [3,2]
 * Total count: 6
 * ```
 *
 * @link https://docs.python.org/3/library/itertools.html#itertools.permutations itertools.permutations (source)
 *
 * @param array $pool
 * @param null|int $r
 *
 * @return \Generator
 */
function permutations($pool, $r = null)
{
    $n = count($pool);
    $r = (null === $r) ? $n : $r;
    if ($r <= $n) {
        $indices = py_range($n);
        $cycles = py_range($n, $n - $r, -1);
        $ret = [];
        foreach (array_slice($indices, 0, $r) as $i) {
            $ret[] = $pool[$i];
            unset($i);
        }
        yield $ret;
        while ($n) {
            $isDone = true;

            foreach (array_reverse(py_range($r)) as $i) {
                $cycles[$i] -= 1;
                if (0 === $cycles[$i]) {
                    $indices = array_merge(
                        array_slice($indices, 0, $i),
                        array_merge(
                            array_slice($indices, $i + 1),
                            array_slice($indices, $i, 1)
                        )
                    );
                    $cycles[$i] = $n - $i;
                } else {
                    $j = $cycles[$i];

                    // swap array values: $i and $tmpKey
                    $tmpValue = $indices[$i];
                    $tmpKey = count($indices) - $j;
                    $indices[$i] = $indices[$tmpKey];
                    $indices[$tmpKey] = $tmpValue;
                    unset($tmp, $y);

                    $ret = [];
                    foreach (array_slice($indices, 0, $r) as $i0) {
                        $ret[] = $pool[$i0];
                    }
                    yield $ret;
                    $isDone = false;
                    break;
                }
            }

            if ($isDone) {
                break;
            }
        }
    }
}