hvad er fejlen i koden

Jeg har følgende linier som giver en warning :


if (isset($HTTP_POST_VARS['userid']) && isset($HTTP_POST_VARS['password']))
{
  // if the user has just tried to log in
  $userid = $HTTP_POST_VARS['userid'];
  $password = $HTTP_POST_VARS['password'];

  $db_conn = mysql_connect("localhost", "root")  or die(mysql_error());
  mysql_select_db('ident', $db_conn)  or die(mysql_error());
  $query = 'select * from ident '
          ."where name='$userid' "
          ." and pass='$password' ";
  $result = mysql_query($query, $db_conn);
  if (mysql_num_rows($result) >0 )
  {

Og linie 16 er denne -
if (mysql_num_rows($result) >0 )
 


Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in c:\apache\apache\htdocs\loginform.php on line 16