出现未知的数组元素？

I was writing one code, when suddenly I've found out, that some strange values appear.

    function purify($output){
    $temp = 0;
    $max_index = count($output);
    for($i=0;$i < $max_index; $i++){
        if(strlen($output[$i]) == 3){
                $str = str_split($output[$i]);
                arsort($str);
                $str = implode($str);
                $output[$temp] = $str;
                $temp++;
        }
        else unset($output[$i]);
    }
    return array_unique($output);
}

When I pass an array consisting from these elements:

11 115 165 138 999 885 999 456 135 726 642 425 426

I get this output:

array(11) { [1]=> string(3) "651" [2]=> string(3) "831" [3]=> string(3) "999" [4]=> string(3) "885" [6]=> string(3) "654" [7]=> string(3) "531" [8]=> string(3) "762" [9]=> string(3) "642" [10]=> string(3) "542" [12]=> string(3) "426" [0]=> string(3) "511" }

How did the 12th element ([12]=> string(3) "426") get there when $temp only went up to 11? I can't get my head around it.

It's not a good practice to change the array $output as you are iterating through it.

Consider creating a new array called $result and change this line:

 $output[$temp] = $str;

to:

 $result[$temp] = $str;

This way you won't touch $output until the end of the loop.

It seems to me that you are seeing weird indexes because your changes of $output influence the next iterations.

Since you are overwriting the parameter output array, the elements initially in the array remain where they are (except if their length is != 3).

Maybe you could try

function purify($output){
    $result = array();
    $max_index = count($output);
    for($i=0;$i < $max_index; $i++){
        if(strlen($output[$i]) == 3){
                $str = str_split($output[$i]);
                arsort($str);
                $result[] = implode($str);
        }
    }
    return array_unique($result);
}

By the way, do you really want to use arsort and not rsort? rsort makes sense since you are using an indexed array (and not an associative array).

                rsort($str);