How to replace the duplicated values in a string by a specific character?

How can I change the value of the duplicated character in a string?

if they are duplicated I want to have a specific value for that else if they are not, I also want to put a specific value for that too.

Scenario 1:
Input: hello

Output: ..??.

Scenario 2:

Input: elephant
Output: ?.?….

$str = "elephant";

$arr= str_split($str);

    for($i = 0; $i < count($arr); $i++) {  
     for($j = $i + 1; $j < count($arr); $j++) {  
        if($arr[$i] == $arr[$j]){
          
       }
     }  
    }

I really don’t know on how will I change the value of it then implode them again.

>Solution :

try this:

$str = "hello";
$arr = str_split($str);
$duplicates = array_unique(array_diff_assoc($arr,array_unique($arr)));
for($i = 0; $i < strlen($str); $i++) {
   if(in_array($str[$i], $duplicates)) {
      $str[$i] = '?';
   } else {
      $str[$i] = '.';
   }
}
echo $str;