It seems like we must use triple colons ":::" at the end of the connection string, such as:
$conn_url = "CUBRID:localhost:33000:demodb:::";
$user = "dba";
$passwd = "";
$conn = cubrid_connect_with_url($conn_url, $user, $passwd);
In this case, the connection is opened successfully
However, the connection should be also opened with a connection string "CUBRID:localhost:33000:demodb"
It is a bug since we don't have any specification that we must use ":::" at the end of the connection string, e.g. http://www.php.net/manual/en/function.cubrid-connect-with-url.php