server.php

<?php
session_start();

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

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

//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']);

  //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($errors, "Password is required"); }
  if ($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
    $queryusers = "INSERT INTO users (username, email, password) VALUES ('$username', '$email', '$password')";
    mysqli_query($db, $queryusers);
    $_SESSION['username'] = $username;
    $_SESSION['success'] = "You are now logged in";
    header('location: index.php');
  }
}
//LOGIN USER
if (isset($_POST['login_user'])) {
  $username = mysqli_real_escape_string($db, $_POST['username']);
  $password = mysqli_real_escape_string($db, $_POST['password']);

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

?>