problemi mysql_num_rows()

[davidspy]

salve mi da questo errore :

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /web/htdocs/www.visualdays.com/home/reportfind.php on line 337

Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /web/htdocs/www.visualdays.com/home/reportfind.php on line 349

il mio codice :

  function GetResource()
        {
            $this->DbConnectAndSelect();
            $sql = "SELECT *, MATCH($this->fulltext) AGAINST('$this->key' IN BOOLEAN MODE) AS tot FROM $this->table WHERE MATCH($this->fulltext) AGAINST('$this->key' IN BOOLEAN MODE) and activa='1' ORDER BY tot DESC";
            $this->res = mysql_query($sql, $this->conn);

        }


 function GetResourcelimit()
        {
            $this->DbConnectAndSelect();
            $sql = "SELECT *, MATCH($this->fulltext) AGAINST('$this->key' IN BOOLEAN MODE) AS tot FROM $this->table WHERE MATCH($this->fulltext) AGAINST('$this->key' IN BOOLEAN MODE) and activa='1' order by data desc LIMIT $primo, $per_page";
            $this->res = mysql_query($sql, $this->conn);

        }


parti di codice 

  $search2 = new Search($key);
$search2->GetResourcelimit();

 riga337---------$num_record = mysql_num_rows($search->res);
 $colonne = 4;
   $tot_righe = $num_record/$colonne;

  $i_x = 0;
$x_x = 0;
$k_x = 0;
  $cell_width = floor(100/$colonne);


 echo '<tr>' ;

riga 349------------while($results = mysql_fetch_array($search->res)) {


non capisco se mischi le class o dipende da altro