具体如下：

 $v)
    {
      $tmp = $index;
      unset($tmp[$i]);    //去掉当前的前缀
      /* 调试信息，便于理解
      echo "len $n , cur $i , index:\n";
      var_dump($tmp);
       */
      $ret = perm($s, $n-1, $tmp);  //递归得到稍短的排列
      if($ret != '')
      {
        foreach($ret as $r)
        {
          $res[] = $s[$v] . $r;  //将稍短的排列逐个拼上当前的前缀
        }
      }
      else
      {
        $res[] = $s[$v];
      }
    }
    return $res;
  }
}
function getPerm($s)
{
  $n = strlen($s);
  $index = range(0, $n-1);
  //得到不同长度的排列
  for($i=1; $i<=$n; $i++)
  {
    var_dump(perm($s, $i, $index));
  }
}
getPerm('abcd');
?>

运行结果：

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"
}