PHP algorithm to generate all combinations of a specific size from a single set

I would use a recursive function. Here's a (working) example with comments. Hope this works for you!

function sampling($chars, $size, $combinations = array()) {

    # if it's the first iteration, the first set 
    # of combinations is the same as the set of characters
    if (empty($combinations)) {
        $combinations = $chars;
    }

    # we're done if we're at size 1
    if ($size == 1) {
        return $combinations;
    }

    # initialise array to put new values in
    $new_combinations = array();

    # loop through existing combinations and character set to create strings
    foreach ($combinations as $combination) {
        foreach ($chars as $char) {
            $new_combinations[] = $combination . $char;
        }
    }

    # call same function again for the next iteration
    return sampling($chars, $size - 1, $new_combinations);

}

// example
$chars = array('a', 'b', 'c');
$output = sampling($chars, 2);
var_dump($output);
/*
array(9) {
  [0]=>
  string(2) "aa"
  [1]=>
  string(2) "ab"
  [2]=>
  string(2) "ac"
  [3]=>
  string(2) "ba"
  [4]=>
  string(2) "bb"
  [5]=>
  string(2) "bc"
  [6]=>
  string(2) "ca"
  [7]=>
  string(2) "cb"
  [8]=>
  string(2) "cc"
}
*/

A possible algorithm would be:

$array_elems_to_combine = array('A', 'B', 'C');
$size = 4;
$current_set = array('');

for ($i = 0; $i < $size; $i++) {
    $tmp_set = array();
    foreach ($current_set as $curr_elem) {
        foreach ($array_elems_to_combine as $new_elem) {
            $tmp_set[] = $curr_elem . $new_elem;
        }
    }
    $current_set = $tmp_set;
}

return $current_set;

Basically, what you will do is take each element of the current set and append all the elements of the element array.

In the first step: you will have as result ('a', 'b', 'c'), after the seconds step: ('aa', 'ab', 'ac', 'ba', 'bb', 'bc', 'ca', 'cb', 'cc') and so on.

You can do this recursively. Note that as per your definition, the "combinations" of length n+1 can be generated from the combinations of length n by taking each combination of length n and appending one of the letters from your set. If you care you can prove this by mathematical induction.

So for example with a set of {A,B,C} the combinations of length 1 are:

A, B, C

The combinations of length 2 are therefore

(A, B, C) + A = AA, BA, CA
(A, B, C) + B = AB, BB, BC
(A, B, C) + C = AC, CB, CC

This would be the code and here on ideone

function comb ($n, $elems) {
    if ($n > 0) {
      $tmp_set = array();
      $res = comb($n-1, $elems);
      foreach ($res as $ce) {
          foreach ($elems as $e) {
             array_push($tmp_set, $ce . $e);
          }
       }
       return $tmp_set;
    }
    else {
        return array('');
    }
}
$elems = array('A','B','C');
$v = comb(4, $elems);

Here is a code made by a friend, it generated unique combinations of X numbers from a list of numbers.

If you have a list of numbers, like 1,3,4,7,12 you can generate sets of X numbers, all unique, no repetitive.

First function works with PHP 7.4 or more, and second uses keys to store the values. Both work very well based on benchmark.

function get_combos74($map, $size, &$generated = [], $loop = 1, $i = 0, $prefix = [])
{
    if ($loop == 1) {
        sort($map);
    }

    for (; $i < count($map); $i++) {
        if ($loop < $size) {
            get_combos74($map, $size, $generated, $loop + 1, $i + 1, [...$prefix, $map[$i]]);
        } else {
            $generated[] = [...$prefix, $map[$i]];
        }
    }

    return $generated;
}
function get_combosSTR($map, $size, &$generated = [], $loop = 1, $i = 0, $prefix = '')
{
    if ($loop == 1) {
        sort($map);
    }

    for (; $i < count($map); $i++) {
        if ($loop < $size) {
            get_combosSTR($map, $size, $generated, $loop + 1, $i + 1, "$prefix{$map[$i]}:");
        } else {
            $generated["$prefix{$map[$i]}"] = 0;
        }
    }

    return $generated;
}