error with the iconv () function in PHP

  

What happens is that the code had written it and everything worked   Correctly. after a few days. enter to verify everything and the   nothing the text that force from UTF 8 to iso 8859-1 did not fulfill its function   I made changes in the database or in the code. if they find another   way to improve this or find the problem thank you very much.

Code: work on MariaDB, Php 7 and Gnu / linux mint (the whole package of xampp, current version.) login from localhost.

  <select name="radicarp">
        <?php 
        while ($row = mysqli_fetch_array($data)){

?>
    <option value="<?php echo $subserie;    ?>"><?php echo $id; echo "&nbsp;&nbsp;"; echo $subserie; echo "&nbsp;&nbsp;";   ?><?php echo iconv(mb_detect_encoding($row['subserie'], mb_detect_order(), true),   "UTF-8", $row['subserie']);    ?></option>  
<?php
      $subserie++;      
   }                                            
$mysqli->close();
?>
    </select>

The error it generates is:

  

notice iconv () detected and illegal character in input string in ....

I want a list with a select to be shown in a .php file without the agreed words that I am calling from the database to be displayed, so that they do not show up with an error.

  

note: already compile on iso 88859-1 utf8 as I said before there was already   worked but from one moment to the next I stop doing it.