试试这段代码吧，解码mime的

function decode_mime_string ($string) {
   $pos = strpos($string, '=?');
   if (!is_int($pos)) {
     return $string;
   }
   
   $preceding = substr($string, 0, $pos); // save any preceding text
   
   $search = substr($string, $pos+2, 75); /* the mime header spec says this is the longest a single encoded word can be */
   $d1 = strpos($search, '?');
   if (!is_int($d1)) {
     return $string;
   }
   
   $charset = substr($string, $pos+2, $d1);
   $search = substr($search, $d1+1);
   
   $d2 = strpos($search, '?');
   if (!is_int($d2)) {
     return $string;
   }
   
   $encoding = substr($search, 0, $d2);
   $search = substr($search, $d2+1);
   
   $end = strpos($search, '?=');
   if (!is_int($end)) {
     return $string;
   }
   
   $encoded_text = substr($search, 0, $end);
   $rest = substr($string, (strlen($preceding . $charset . $encoding . $encoded_text)+6));
   
   switch ($encoding) {
   case 'Q':
   case 'q':
     $encoded_text = str_replace('_', '%20', $encoded_text);
     $encoded_text = str_replace('=', '%', $encoded_text);
     $decoded = urldecode($encoded_text);
     
     if (strtolower($charset) == 'windows-1251') {
       $decoded = convert_cyr_string($decoded, 'w', 'k');
     }
     break;
     
   case 'B':
   case 'b':
     $decoded = urldecode(base64_decode($encoded_text));
     
     if (strtolower($charset) == 'windows-1251') {
       $decoded = convert_cyr_string($decoded, 'w', 'k');
     }
     break;
     
   default:
     $decoded = '=?' . $charset . '?' . $encoding . '?' . $encoded_text . '?=';
     break;
   }
   
   return $preceding . $decoded . decode_mime_string($rest);
} // decode_mime_string()
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