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

i trying deduce algorithm generates possible combinations of specific size function accepts array of chars , size parameter , return array of combinations.

example: let have set of chars: set = {a,b,c}

a) possible combinations of size 2: (3^2 = 9)

aa, ab, ac ba, bb, bc ca, cb, cc 

b) possible combinations of size 3: (3^3 = 27)

aaa, aab, aac, aba, abb, acc, caa, baa, bac, .... ad on total combinations = 27 

please note pair size can greater total size of pouplation. ex. if set contains 3 characters can create combination of size 4.

edit: note different permutation. in permutation cannot have repeating characters example aa cannot come if use permutation algorithm. in statistics known sampling.

i use recursive function. here's (working) example comments. hope works you!

function sampling($chars, $size, $combinations = array()) {      # if it's first iteration, first set      # of combinations same set of characters     if (empty($combinations)) {         $combinations = $chars;     }      # we're done if we're @ size 1     if ($size == 1) {         return $combinations;     }      # initialise array put new values in     $new_combinations = array();      # loop through existing combinations , character set create strings     foreach ($combinations $combination) {         foreach ($chars $char) {             $new_combinations[] = $combination . $char;         }     }      # call same function again next iteration     return sampling($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" } */