server.php

<?php

session_start();

$email="";
$username="";

$errors = array();

$db=mysqli_connect('localhost','root','','SFDatabase') or die("Couldn't connect to database");

$username= mysqli_real_escape_string($db,$_POST['username']);
$email= mysqli_real_escape_string($db,$_POST['email']);
$pwd1= mysqli_real_escape_string($db,$_POST['pwd1']);
$pwd2= mysqli_real_escape_string($db,$_POST['pwd2']);

//validation
if(empty($email)) array_push($errors, "Please enter an email!");
if(empty($username)) array_push($errors, "Please enter a username!");
if(empty($pwd1)) array_push($errors, "Please enter a password!");
if($pwd1 != $pwd2) array_push($errors, "Passwords don't match!");

$user_check_query = "SELECT * FROM users WHERE username = '$username' or email = '$email' LIMIT 1";

$results = mysqli_query($db,$user_check_query);
$user=mysqli_fetch_assoc($results);
if($user){
    if($user['username']===$username) {array_push($errors, "Username already exists!");}
if($user['email']===$email) {array_push($errors, "E-mail already exists!");}
}


if(count($errors)===0){
    $password=md5($pwd1);
    $query="INSERT INTO users (username, password, email) VALUES ('$username', '$password', '$email')";
    mysqli_query($db,$query);

    $_SESSION['username']=$username;
    $_SESSION['success']="You are now logged in";
    header('location: index.php');
}

if(isset($_POST['login_user'])){
    $username=mysqli_real_escape_string($db,$_POST['username']);
    $password=mysqli_real_escape_string($db,$_POST['pwd1']);

    if(empty($username)){
        array_push($errors,"Please enter a username!");
    }
    if(empty($password)){
        array_push($errors,"Please enter a password!");
    }
    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_results($results)){
            $_SESSION['$username']=$username;
            $_SESSION['success']="Logged in successfully";
            header('location: index.php');
        }else{
            array_push($errors,"Wrong credentials, please try again!");
        }
    }
}
?>