server.php

<?php
session_start();

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

// connect to the database
$db = mysqli_connect('localhost', getenv('USER_NAME'), getenv('DB_PASSWORD'), 'hydroponics');

// REGISTER USER
if (isset($_POST['reg_user'])) {
  // receive all input values from the form
  $firstname = mysqli_real_escape_string($db, $_POST['firstname']);
  $lastname = mysqli_real_escape_string($db, $_POST['lastname']);
  $email = mysqli_real_escape_string($db, $_POST['email']);
  $password = mysqli_real_escape_string($db, $_POST['password']);

  // form validation: ensure that the form is correctly filled ...
  // by adding (array_push()) corresponding error unto $errors array
  if (empty($firstname)) { array_push($errors, "Firstname is required"); }
  if (empty($lastname)) { array_push($errors, "Lastname is required"); }
  if (empty($email)) { array_push($errors, "Email is required"); }
  if (empty($password)) { array_push($errors, "Password is required"); }

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

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

// LOGIN USER
if (isset($_POST['login_user'])) {
    $email = mysqli_real_escape_string($db, $_POST['email']);
    $password = mysqli_real_escape_string($db, $_POST['password']);
  
    if (empty($email)) {
        array_push($errors, "Email is required");
    }
    if (empty($password)) {
        array_push($errors, "Password is required");
    }
  
    if (count($errors) == 0) {
      $password = md5($password);
      $query = "SELECT * FROM users WHERE email='$email' AND password='$password'";
      $results = mysqli_query($db, $query);
      if (mysqli_num_rows($results) == 1) {
        $_SESSION['username'] = $email;
        $_SESSION['success'] = "You are now logged in";
        header('location: ajax.php');
      } else {
          array_push($errors, "Wrong email/password combination");
      }
    }
  }
  
  ?>