function encode_string ($str, $encoding = "base64") {
      switch(strtolower($encoding)) {
         case "base64":
            // chunk_split is found in PHP >= 3.0.6
            $encoded = chunk_split(base64_encode($str));
            break;

         case "7bit":
         case "8bit":
            $encoded = $this->fix_eol($str);
            if (substr($encoded, -2) != "\r\n")
               $encoded .= "\r\n";
            break;

         case "binary":
            $encoded = $str;
            break;

         case "quoted-printable":
            $encoded = $this->encode_qp($str);
            break;

         default:
            $this->error_handler(sprintf("Unknown encoding: %s", $encoding));
            return false;
      }
      return($encoded);
   }