dove azzo sbaglio ? la query? sto impazzendo

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /var/www/jsco0026/php/filtra.php on line 10
Ci sono risultati
Warning: mysql_fetch_row(): supplied argument is not a valid MySQL result resource in /var/www/jsco0026/php/filtra.php on line 13


<?php
$id = $HTTP_GET_VARS["filtra2"];

$conn = mysql_connect("localhost","xxx","xxx") or die(mysql_error());
mysql_select_db("xxx",$conn) or die(mysql_error());

$result = mysql_query ( "SELECT * FROM menu_schede WHERE cognome=$filtra2 or passaporto=$filtra2 or documento=$filtra2 or user=$filtra2 or hotel=$filtra2 or data=$filtra2" );

$num_rows = mysql_num_rows( $result );
echo "Ci sono $num_rows risultati";
echo "<table>";
while ( $a_row = mysql_fetch_row( $result ) )
{
echo "<tr>";
foreach ( $a_row as $field )
echo "<td>$field</td>";
echo "</tr>";
}
echo "</table>";
mysql_close ( $conn );
echo "</body>";
?>