public  function addCharsAt(int $index,string $char){
        $str='';
        for ($i=0;$i<$this->length;) {
            if ($i == $index)
                $str .= $char;
            else {
                $str .= $this->string[$i];
                $i++;
            }
        }
        $this->length++;
        $this->string=&$str;
    }