function permutations($letters,$num)
{
   $last = str_repeat($letters[0],$num);
   $result = array();
   while($last != str_repeat($this->lastchar($letters),$num))
   {
       $result[] = $last;
       $last = $this->char_add($letters,$last,$num-1);
   }
   $result[] = $last;
   return $result;
}

function char_add($digits,$string,$char)
{
    error_reporting(0);

    if($string[$char] < $this->lastchar($digits))
    {
       $string[$char] = $digits[strpos($digits,$string[$char])+1];
       return $string;
    }
    else
    {
       $string = $this->changeall($string,$digits[0],$char);
       return $this->char_add($digits,$string,$char-1);
    }
}

function lastchar($string)
{
   return $string[strlen($string)-1];
}

function changeall($string,$char,$start = 0,$end = 0)
{
   if($end == 0) $end = strlen($string)-1;
   for($i=$start;$i<=$end;$i++){
       $string[$i] = $char;
   }
   return $string;
}

print_r($this->permutations('1234',2));

Array(
[0] => 11
[1] => 12
[2] => 13
[3] => 14
[4] => 21
[5] => 22
[6] => 23
[7] => 24
[8] => 31
[9] => 32
[10] => 33
[11] => 34
[12] => 41
[13] => 42
[14] => 43
[15] => 44)