server.php

<?php
    session_start();

    // variable declaration
    $username = "";
    $email    = "";
    $errors = array();
    $_SESSION['success'] = "";

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

    // 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
        if (empty($username)) {
        //array_push($errors, "Username is required");
        print("POST['username'] IS empty!");
        }
        if (empty($email)) {
        //array_push($errors, "Email is required");
        print("POST['email'] IS empty!");
        }
        if (empty($password_1)) {
        //array_push($errors, "Password is required");
        print("POST['pass'] IS empty!");
        }

        if ($password_1 != $password_2) {
            array_push($errors, "The two passwords do not match");
        }

        // 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
            $query = "INSERT INTO users (username, email, password)
                      VALUES('$username', '$email', '$password')";
            mysqli_query($db, $query);

            $_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");
            }
        }
    }

?>