Ez a hibaüzenet mit jelent? Warning: mysql_num_rows () : supplied argument is not a valid MySQL result resource in /home/a4755475/public_html/login. Php on line 203

$host=""; // Host name

$username=""; // Mysql username

$password=""; // Mysql password

$db_name=""; // Database name

$tbl_name="user"; // Table name

// Connect to server and select databse.

mysql_connect("$host", "$username", "$password")or die("cannot connect");

mysql_select_db("$db_name")or die("cannot select DB");

// username and password sent from form

$myusername=$_POST['login_username'];

$mypassword=$_POST['login_password'];

// To protect MySQL injection (more detail about MySQL injection)

$myusername = stripslashes($myusername);

$mypassword = stripslashes($mypassword);

$myusername = mysql_real_escape_string($myusername);

$mypassword = mysql_real_escape_string($mypassword);

$sql="SELECT * FROM $tbl_name WHERE username='$myusername' and password='$mypassword'";

$result=mysql_query($sql);

// Mysql_num_row is counting table row

$count=mysql_num_rows($result);

// If result matched $myusername and $mypassword, table row must be 1 row

if($count==1){

// Register $myusername, $mypassword and redirect to file "login_success.php"

session_register("myusername");

session_register("mypassword");

header("Location:login_success.php");

}

else {

echo "Wrong Username or Password";

}