避免重新转换UTF-8字符串PHP

Well, I have a BD with a lot of ISO strings and another with UTF-8 (yes, I ruin everything) and now I'm making a custom function that rewrite all the BD again to have all in UTF-8, the problem, is the conversion with UTF-8 strings... The ? appears:

$field = $fila['Field'];
            $acon = mysql_fetch_array(mysql_query("SELECT `$field` as content FROM `$curfila` WHERE id='$i'"));
            $content = $acon['content'];
            if(!is_numeric($content)) {
                if($content != null) {
                    if(ip2long($content) === false) {
                        mb_internal_encoding('UTF-8');
                        if(mb_detect_encoding($content) === "UTF-8") {
                            $sanitized = utf8_decode($content);
                            if($sanitized != $content) {
                                echo 'Fila [ID ('.$i.')] <b>'.$field.'</b> => '.$sanitized.'<br>';
                                //mysql_query("UPDATE `$curfila` SET `$field`='$sanitized' WHERE id='$i'");
                            }
                        }
                    }
                }
            }

PD: I check all the columns and rows of all the tables of the BD. (I show all everything before doing anything)

So, how can I detect that?

I tried mb_detect_encoding, but the all the string are in UTF-8... So, which function can I use now?

Thanks in advance.