<?php
$arr = array('a','b','c','d');
$result = array();
$t = getCombinationToString($arr, 1);
print_r($t);
$t = getCombinationToString($arr, 2);
$t2 = getunique($t);
print_r($t2);
$t = getCombinationToString($arr, 3);
$t2 = getunique($t);
print_r($t2);
$t = getCombinationToString($arr, 4);
$t2 = getunique($t);
print_r($t2);

function getunique($t){
 $t2 = array();
 //print_r($t);
 for($i=0;$i<count($t);$i++){
  $count_list = array_count_values($t[$i]);
  $flag = 1;
  foreach($count_list as $ck=>$cv){
   if($cv>1){
    $flag = 0;
    break;
   }
  }
  if($flag){
   sort($t[$i]);
   $flag2 = 1;
   if($t2){
    foreach($t2 as $t2k=>$t2v){
     if($t[$i]==$t2v){
      $flag2 = 0;
      break;
     }
    }
   }
   if($flag2){
    $t2[] = $t[$i];
   }
  }
 }
 return $t2;
}

function getCombinationToString($arr, $m) {
 if ($m ==1) {
  return $arr;
 }
 $result = array();
 
 $tmpArr = $arr;
 unset($tmpArr[0]);
 for($i=0;$i<count($arr);$i++) {
  $s = $arr[$i];
  $ret = getCombinationToString(array_values($tmpArr), ($m-1), $result);
  
  foreach($ret as $row) {
   //$result[] = $s . $row;
   $temp = array();
   $temp[] = $s;
   if(is_array($row)){
    $temp = array_merge($temp,$row);
   }else{
    $temp[] = $row;
   }
   sort($temp);
   $result[] = $temp;
  }
 }
 return $result;
}

?>