codice:
<?php  
$conn=mysql_connect("localhost", "root", "root");  
if (!$conn)  
{ 	 
die ("Connessione al db fallita!!! ".mysql_error());  
}  
$db=mysql_select_db ("prova", $conn);  
session_start ();   
if ((!isset($_POST['utente'])) && (!isset($_POST['password'])))   
{  
//  form   
echo "<form method=\"post\" action=\"".$_SERVER['PHP_SELF']."\">    
User: <input type=\"text\" name=\"username\">
    
Password_: <input type=\"password\" name=\"password\">
    
<input type=\"submit\" value=\"Invia\"></form>";    
}
else
{  
// query in tabella   
$sql = mysql_query("SELECT * FROM utenti WHERE username='".$_POST['username']."' AND password='".$_POST['password']."'");   
if (mysql_num_rows($sql)>0)   
{   
$_SESSION['username']=$_POST['username'];    $_SESSION['password']=$_POST['password'];    header("Location: pagina_riservata.php");   }  
else 
{
echo "Login errato";
}    
} 
?>
ma mi da errore sotto alla linea dell'if
mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\xampp\htdocs\php\for.php on line 21