var loginData, urlPath;

// Allow users to log in or register
function Login() {

    if(!document.getElementById("usernameField")) { // If we have no username field on this page, we are just logging in
        loginData = "email=" + $("#emailField").val() + "&password=" + $("#passwordField").val() + "&action=" + "loggingIn";
        urlPath = "index.php";
    } else { // we are registering
        loginData = "username=" + $("#usernameField").val() + "&email=" + $("#emailField").val() + "&password=" + $("#passwordField").val() + "&action=" + "register";
        urlPath = "../index.php";
    }

    // Send the login/registration data to database
    $(document).ready(function() {
        $.ajax({
            type: "POST",
            url: urlPath,
            data: loginData,
            dataType: 'JSON',
            success: function (result) {
                alert(result); // never called

                if(result.returnVal=="registered") {
                    document.getElementById('notification').innerHTML = "You have been registered";
                } else if (result.returnVal=="username") {
                    document.getElementById('notification').innerHTML = "Username already taken";
                } else if (result.returnVal=="email") {
                    document.getElementById('notification').innerHTML = "Email already taken";
                } else if (result.returnVal=="notRegistered") {
                    document.getElementById('notification').innerHTML = "Please enter registered email";
                } else if (result.returnVal=="loginFail") {
                    document.getElementById('notification').innerHTML = "Please enter correct password";
                } else if (result.returnVal=="loggedIn") {
                    $('#myModal').modal('hide');
                    document.getElementById('loginButton').innerHTML = "Account Settings";
                } else { // Something wrong, tell us
                    alert(result);
                }
            }
        })
    })
}

<?php
    require_once("configuration.php");

    class Model {

        private $registered;
        private $loggedIn;

        function __construct() {
            $this->registered = false;
            $this->loggedIn = false;
        }

        function Register($username, $email, $password) {
            global $dbCon;

            // Make sure username isnt taken
            $usernameValue = $this->getValueFromDB('users', $username, 'username');

            if ($usernameValue != null) { // Username in DB: Tell client that username is taken
            error_log("username");
                $returned = array('returnVal' => "username");
                echo json_encode($returned);
                //return "username";

            } else { // Username is not yet taken

                // Make sure email isnt used by another user
                $emailValue = $this->getValueFromDB('users', $email, 'email');

                if ($emailValue != null) { // Email in DB: Tell client that email is taken
                error_log("email");
                    $returned = array('returnVal' => "email");
                    echo json_encode($returned);
                    //return "email";

                } else { // Email is not yet taken

                    $sql = "INSERT INTO users (username,email,password) VALUES (:username,:email,:password)";

                    $stmt = $dbCon->prepare($sql);

                    $stmt->bindParam(':username', $username, PDO::PARAM_STR);
                    $stmt->bindParam(':email', $email, PDO::PARAM_STR);
                    $stmt->bindParam(':password', $password, PDO::PARAM_STR);
                    $stmt->execute(array(':username' => $username,':email' => $email,':password' => $password));

                    if($stmt == false) {
                        error_log("Failed to put registration info in DB");
                    } else {
                        error_log("Registered");
                        $returned = array('returnVal' => "registered");
                        echo json_encode($returned);
                        //return "registered";
                    }
                }
            }
        }