A nice way to decode strings is to use the mb_list_encodings(), but I had two problems with it: 
sometimes, the charset is uppercase in the header an lowercase in mb_list_encodings() and sometimes, the charset is not in the mb_list_encodings() list.
<?php
function upperListEncode() { $encodes=mb_list_encodings();
    foreach ($encodes as $encode) $tencode[]=strtoupper($encode);
    return $tencode;
    }
function decode($string) {
    $tabChaine=imap_mime_header_decode($string);
    $texte='';
    for ($i=0; $i<count($tabChaine); $i++) {
        
        switch (strtoupper($tabChaine[$i]->charset)) { case 'UTF-8': $texte.= $tabChaine[$i]->text; break;
            case 'DEFAULT': $texte.= $tabChaine[$i]->text; break;
            default: if (in_array(strtoupper($tabChaine[$i]->charset),upperListEncode())) {$texte.= mb_convert_encoding($tabChaine[$i]->text,'UTF-8',$tabChaine[$i]->charset);}
                     else { $ret = iconv($tabChaine[$i]->charset, "UTF-8", $tabChaine[$i]->text);    
                          if (!$ret) $texte.=$tabChaine[$i]->text;  else $texte.=$ret;
                        }                    
                break;
            }
        }
        
    return $texte;    
    }
?>