PHP login form error

Try using exit after header functions because after redirect without exit, script will continue with execution:

<?php
session_start(); // Starting Session
$error=''; // Variable To Store Error Message
if (isset($_POST['submit'])) {
if (empty($_POST['username']) || empty($_POST['password'])) {
$error = "חלק מהנתונים שסופקו, שגויים.";
}
else
{
// Define $username and $password
$username=$_POST['username'];
$password=$_POST['password'];
// Establishing Connection with Server by passing server_name, user_id and password as a parameter
include "config.php";
// To protect MySQL injection for Security purpose
$username = stripslashes($username);
$password = stripslashes($password);
$username = mysql_real_escape_string($username);
$password = mysql_real_escape_string($password);
// Selecting Database
$tbl_name="users";
//$db = mysql_select_db($tbl_name, $conn);
// SQL query to fetch information of registerd users and finds user match.
$query = mysql_query("select * from $tbl_name where userpassword='$password' AND username='$username'", $conn);
$rows = mysql_num_rows($query);
$dbdata = mysql_fetch_array($query) or die(mysql_error());

if ($rows == 1) {
$flag = $dbdata['admin'];
if ($flag == 1) {
$_SESSION['login_user']=$username; // Initializing Session
header("location: index.php");exit; // Redirecting To Other Page
} elseif($flag == 0){
$_SESSION['login_user']=$username; // Initializing Session
header("location: user.php");exit; // Redirecting To Other Page
} else{
session_destroy();
header("location: errorlog.php");exit;
}}
mysql_close($conn); // Closing Connection
}}
?>

Also make sure you have error reporting enabled in your script:

<?php
// Turn off error reporting
error_reporting(0);

// Report runtime errors
error_reporting(E_ERROR | E_WARNING | E_PARSE);

// Report all errors
error_reporting(E_ALL);

// Same as error_reporting(E_ALL);
ini_set("error_reporting", E_ALL);

// Report all errors except E_NOTICE
error_reporting(E_ALL & ~E_NOTICE);
?>

where you have an incorrect username/password

if ($rows == 1) {

will be false. You have caught an incorrect flag but there is no else on this if statement.

if you are expecting it to destroy the session and head to the error page add

session_destroy();
header("location: errorlog.php");

into an else for that group. Otherwise writing to the $error variable such as:

$error='username or password not recognised';

Your error check for checking if the user does not exist is wrong. the mysql_num_rows($query) returns FALSE if a user does not exist. Either an extra else statement after your if (row == 1). OR check if $rows === false and redirect accordingly.

So adding this before your if should suffice. (insert before the if(row==1))

if($rows === false){
    session_destroy();
    header("location: errorlog.php");
}