<?php
session_start();

// initializing variables
$username = "";
$email    = "";
$errors = array();

// connect to the database
$db = mysqli_connect('localhost', 'root', 'test', 'test');

// REGISTER USER
if (isset($_POST['reg_user'])) {
  // receive all input values from the form
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $email = mysqli_real_escape_string($db, $_POST['email']);
  $password_1 = mysqli_real_escape_string($db, $_POST['password_1']);
  $password_2 = mysqli_real_escape_string($db, $_POST['password_2']);
  $balance = mysqli_real_escape_string($db, $_POST['balance']);

  // form validation: ensure that the form is correctly filled ...
  // by adding (array_push()) corresponding error unto $errors array
  if (empty($username)) { array_push($errors, "Username is required"); }
  if (empty($email)) { array_push($errors, "Email is required"); }
  if (empty($password_1)) { array_push($errors, "Password is required"); }
  if (empty($balance)) { $balance = "0"; }
  if ($password_1 != $password_2) {
    array_push($errors, "The two passwords do not match");
  }

  // first check the database to make sure
  // a user does not already exist with the same username and/or email
  $user_check_query = "SELECT * FROM users WHERE username='$username' OR
email='$email' LIMIT 1";
  $result = mysqli_query($db, $user_check_query);
  $user = mysqli_fetch_assoc($result);

  if ($user) { // if user exists
    if ($user['username'] === $username) {
      array_push($errors, "Username already exists");
    }

    if ($user['email'] === $email) {
      array_push($errors, "email already exists");
    }
  }

  // Finally, register user if there are no errors in the form
  if (count($errors) == 0) {
    $password = md5($password_1);//encrypt the password before saving in the database

$query = "INSERT INTO users (username, email, password, balance)
          VALUES('$username', '$email', '$password', '$balance')";
mysqli_query($db, $query);
$_SESSION['username'] = $username;
$_SESSION['balance'] = $balance;
$_SESSION['success'] = "You are now logged in";
header('location: index.php');
  }
}

// ...

// LOGIN USER
if (isset($_POST['login_user'])) {
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $password = mysqli_real_escape_string($db, $_POST['password']);
  $balance = mysqli_real_escape_string($db, $_POST['balance']);



  if (empty($username)) {
    array_push($errors, "Username is required");
  }
  if (empty($password)) {
    array_push($errors, "Password is required");
  }

  if (count($errors) == 0) {
    $password = md5($password);
    $query = "SELECT * FROM users WHERE username='$username' AND
password='$password'";




    $results = mysqli_query($db, $query);
    if (mysqli_num_rows($results) == 1) {
      $_SESSION['username'] = $username;
      $_SESSION['password'] = $password;
  $_SESSION['balance'] = $balance;
  $_SESSION['success'] = "You are now logged in";
  header('location: index.php');
}else {
    array_push($errors, "Wrong username/password combination");
}
  }
}

?>

this is the code on how to query data on db right?
$query = "SELECT * FROM users WHERE username='$username' AND
    password='$password'";