2進数を使って全ての組み合わせを取得する関数。A function to get all combinations using binary numbers.

getAllCombinations.php

<?php

/**
 * Code to get all combinations of array elements
 */

namespace Macocci7;

class Combination
{
    /**
     * returns system bit
     * @param
     * @return  int
     */
    public static function systemBit(): int
    {
        // PHP_INT_ISZE:
        // - 32bit-system: 4 (bytes)
        // - 64bit-system: 8 (bytes)
        return PHP_INT_SIZE * 8;
    }

    /**
     * returns all combinations
     * @param   array   $items
     * @param   bool    $sort = false
     * @return  array
     */
    public function all(array $items, bool $sort = false)
    {
        $count = count($items);
        if (0 === $count) {
            throw new \Exception("Empty array set.");
        }
        if ($count >= $this->systemBit() - 1) {
            throw new \Exception("Too many elements.");
        }
        $numberOfAllPatterns = 2 ** $count;
        $format = '%0' . $count . 'b';
        $combinations = [];
        for ($i = $numberOfAllPatterns - 1; $i > 0; $i--) {
            $combination = [];
            foreach (str_split(sprintf($format, $i)) as $index => $bit) {
                if ((bool) $bit) {
                    $combination[] = $items[$index];
                }
            }
            $combinations[] = $combination;
        }
        if ($sort) {
            $strs = array_map(fn ($c): string => implode(',', $c), $combinations);
            array_multisort($strs, SORT_ASC, SORT_STRING, $combinations);
        }
        return $combinations;
    }
}

// Starting operation
$start = microtime(true);
$items = ['A', 'B', 'C', 'D', 'E'];
$sort = false;
$c = new Combination();
foreach ($c->all($items, $sort) as $index => $combination) {
    echo sprintf("(%s)\n", implode(', ', $combination));
}
echo sprintf(
    "Time: %.6f sec / Peak Memory: %.3f MB\n",
    microtime(true) - $start,
    memory_get_peak_usage(true) / 1024 / 1024
);

This code results in:

$ php -f getAllCombinations.php 
(A, B, C, D, E)
(A, B, C, D)
(A, B, C, E)
(A, B, C)
(A, B, D, E)
(A, B, D)
(A, B, E)
(A, B)
(A, C, D, E)
(A, C, D)
(A, C, E)
(A, C)
(A, D, E)
(A, D)
(A, E)
(A)
(B, C, D, E)
(B, C, D)
(B, C, E)
(B, C)
(B, D, E)
(B, D)
(B, E)
(B)
(C, D, E)
(C, D)
(C, E)
(C)
(D, E)
(D)
(E)
Time: 0.000265 sec / Peak Memory: 2.000 MB

Changing the sorting flag

$sort = true;

results in:

$ php -f getAllCombinations.php 
(A)
(A, B)
(A, B, C)
(A, B, C, D)
(A, B, C, D, E)
(A, B, C, E)
(A, B, D)
(A, B, D, E)
(A, B, E)
(A, C)
(A, C, D)
(A, C, D, E)
(A, C, E)
(A, D)
(A, D, E)
(A, E)
(B)
(B, C)
(B, C, D)
(B, C, D, E)
(B, C, E)
(B, D)
(B, D, E)
(B, E)
(C)
(C, D)
(C, D, E)
(C, E)
(D)
(D, E)
(E)
Time: 0.000461 sec / Peak Memory: 2.000 MB