An Example of Generating Permutation Algorithm Implemented by php

In this paper, an example is given to describe the generation and permutation algorithm implemented by php. Share it for your reference, as follows:

<?php
function perm($s, $n, $index)
{
  if($n == 0)
  {
     return '';
  }
  else
  {
    $nIndex = count($index);  // Available string subscripts 
    $res = array();
    foreach($index as $i => $v)
    {
      $tmp = $index;
      unset($tmp[$i]);    // Remove the current prefix 
      /*  Debug information for easy understanding 
      echo "len $n , cur $i , index:\n";
      var_dump($tmp);
       */
      $ret = perm($s, $n-1, $tmp);  // Recursion yields a slightly shorter permutation 
      if($ret != '')
      {
        foreach($ret as $r)
        {
          $res[] = $s[$v] . $r;  // Spell the shorter permutations one by one with the current prefix 
        }
      }
      else
      {
        $res[] = $s[$v];
      }
    }
    return $res;
  }
}
function getPerm($s)
{
  $n = strlen($s);
  $index = range(0, $n-1);
  // Get permutations of different lengths 
  for($i=1; $i<=$n; $i++)
  {
    var_dump(perm($s, $i, $index));
  }
}
getPerm('abcd');
?>

Run results:

array(4) {
[0]= >
string(1) "a"
[1]= >
string(1) "b"
[2]= >
string(1) "c"
[3]= >
string(1) "d"
}
array(12) {
[0]= >
string(2) "ab"
[1]= >
string(2) "ac"
[2]= >
string(2) "ad"
[3]= >
string(2) "ba"
[4]= >
string(2) "bc"
[5]= >
string(2) "bd"
[6]= >
string(2) "ca"
[7]= >
string(2) "cb"
[8]= >
string(2) "cd"
[9]= >
string(2) "da"
[10]= >
string(2) "db"
[11]= >
string(2) "dc"
}
array(24) {
[0]= >
string(3) "abc"
[1]= >
string(3) "abd"
[2]= >
string(3) "acb"
[3]= >
string(3) "acd"
[4]= >
string(3) "adb"
[5]= >
string(3) "adc"
[6]= >
string(3) "bac"
[7]= >
string(3) "bad"
[8]= >
string(3) "bca"
[9]= >
string(3) "bcd"
[10]= >
string(3) "bda"
[11]= >
string(3) "bdc"
[12]= >
string(3) "cab"
[13]= >
string(3) "cad"
[14]= >
string(3) "cba"
[15]= >
string(3) "cbd"
[16]= >
string(3) "cda"
[17]= >
string(3) "cdb"
[18]= >
string(3) "dab"
[19]= >
string(3) "dac"
[20]= >
string(3) "dba"
[21]= >
string(3) "dbc"
[22]= >
string(3) "dca"
[23]= >
string(3) "dcb"
}
array(24) {
[0]= >
string(4) "abcd"
[1]= >
string(4) "abdc"
[2]= >
string(4) "acbd"
[3]= >
string(4) "acdb"
[4]= >
string(4) "adbc"
[5]= >
string(4) "adcb"
[6]= >
string(4) "bacd"
[7]= >
string(4) "badc"
[8]= >
string(4) "bcad"
[9]= >
string(4) "bcda"
[10]= >
string(4) "bdac"
[11]= >
string(4) "bdca"
[12]= >
string(4) "cabd"
[13]= >
string(4) "cadb"
[14]= >
string(4) "cbad"
[15]= >
string(4) "cbda"
[16]= >
string(4) "cdab"
[17]= >
string(4) "cdba"
[18]= >
string(4) "dabc"
[19]= >
string(4) "dacb"
[20]= >
string(4) "dbac"
[21]= >
string(4) "dbca"
[22]= >
string(4) "dcab"
[23]= >
string(4) "dcba"
}

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