function broke after updating PHP

I have a little problem with a function which doesn't seem to fully work in new php version and I receive

Notice: String offset cast occurred in D:\xampp\htdocs\decode\bencoded.php on line 266
Notice: String offset cast occurred in D:\xampp\htdocs\decode\bencoded.php on line 270

Here is my function:

function bdecode($s, &$pos=0) {
  if($pos>=strlen($s)) {
    return null;
  }
  switch($s[$pos]){
  case 'd':
    $pos++;
    $retval=array();
    while ($s[$pos]!='e'){
      $key=bdecode($s, $pos);
      $val=bdecode($s, $pos);
      if ($key===null || $val===null)
        break;
      $retval[$key]=$val;
    }
    $retval["isDct"]=true;
    $pos++;
    return $retval;

  case 'l':
    $pos++;
    $retval=array();
    while ($s[$pos]!='e'){
      $val=bdecode($s, $pos);
      if ($val===null)
        break;
      $retval[]=$val;
    }
    $pos++;
    return $retval;

  case 'i':
    $pos++;
    $digits=strpos($s, 'e', $pos)-$pos;
    // Proger_XP: changed (int) -> (float) to avoid trimming of values exceeding
    //            signed int's max value (2147483647).
    $val=(float)substr($s, $pos, $digits);
    $pos+=$digits+1;
    return $val;

//  case "0": case "1": case "2": case "3": case "4":
//  case "5": case "6": case "7": case "8": case "9":
  default:
    $digits=strpos($s, ':', $pos)-$pos;
    if ($digits<0 || $digits >20)
      return null;
    $len=(float)substr($s, $pos, $digits);
    $pos+=$digits+1;
    $str=substr($s, $pos, $len);
    $pos+=$len;
    //echo "pos: $pos str: [$str] len: $len digits: $digits\n";
    return (string)$str;
  }
  return null;
}

i understand that i get a warning in the new php, but i have no idea how to fix it.

line 266 (before case 'd'): switch($s[$pos]){

line 270 (after case '1'): while ($s[$pos]!='e'){