function crea_password($len) {
  $chars = "abcdefghijkmnopqrstuvwxyz023456789";
  srand((double)microtime()*1000000);
  $i = 0;
  $pass = '' ;
  while ($i < $len) {
    $num = rand() % 33;
    $tmp = substr($chars, $num, 1);
    $pass = $pass . $tmp;
    $i++;
  }
  return $pass;
}