protected function makeReplyChar($prevChars = null)
{
$std = '0123456789abcdefghijklmnopqrstuvwxyz';
$std = str_split($std, 1);
if ($prevChars === null) {
return str_repeat($std[0], self::getReplyCharLen());
}
if ($prevChars[strlen($prevChars) - 1] == end($std)) {
if (strlen($prevChars) < 2) {
throw new ReplyLimitationException();
}
reset($std);
$new = $this->makeReplyChar(substr($prevChars, 0, strlen($prevChars) - 1)) . current($std);
} else {
$key = array_search($prevChars[strlen($prevChars) - 1], $std);
$new = substr($prevChars, 0, strlen($prevChars) - 1) . $std[$key + 1];
}
return $new;
}