PHP mysql_num_rows() Returns error
$username = mysql_real_escape_string($username);
$query = "SELECT * FROM users WHERE username = '$username';";
$result = mysql_query($query,$conn);
if(mysql_num_rows($result)>0) //user exists
{
    header('Location: index.php');
    $_SESSION['reg_error']='User already exists';
    die();
}
else
{
$query = "INSERT INTO users ( username, password, salt )
        VALUES ( '$username' , '$hash' , '$salt' );";
mysql_query($query);
mysql_close();
header('Location: index.php');

mysql_num_rows(): supplied argument is not a valid MySQL result resource in [dirctory name]

$query = "SELECT * FROM users WHERE username = '$username';";

$query = "SELECT * FROM users WHERE username = '"
    . mysql_real_escape_string($username)  . "';";

$username = mysql_real_escape_string($username);

$username = mysql_real_escape_string($_POST[$username]);