function _(id) {
    return document.getElementById(id);
}

function emptyElem(x, y) {
    _(x).innerHTML = "";
    _(y).innerHTML = "";
}

function restrict(elem) {
    var getId = _(elem);
    var regEx = new RegExp;
    if(elem == 'username') {
        regEx = /[^a-z0-9]/gi;
    } else if(elem == 'email') {
        regEx = /[' "]/g;
    } else if(elem == 'phoneNum') {
        regEx = /[^0-9]/g;
    }
    getId.value = getId.value.replace(regEx, "");
}

function checkusername() {
    var username = _('username').value;
    if(username != "") {
        _('usernameStatus').innerHTML = "<p class='green'>Checking...</p>";
        var ajax = ajaxObj("POST", "includes/signup_inc.php");
        ajax.onreadystatechange = function() {
            if(ajaxReturn(ajax) == true) {
                _('usernameStatus').innerHTML = ajax.responseText;
            }
        }
        ajax.send("checkusername="+username);
    }
}

function signup() {
    var firstname = _('firstname').value;
    var lastname = _('lastname').value;
    var username = _('username').value;
    var email = _('email').value;
    var gender = _('gender').value;
    var phoneNum = _('phoneNum').value;
    var password = _('password').value;
    var confirmPass = _('confirmPass').value;
    var status = _('status');

    if(firstname == "" || lastname == "" || username == "" || email == "" || gender == "" || phoneNum == "" || password == "" || confirmPass == "") {
        status.innerHTML = "<p class='red'>Please fill out all fields!</p>";
    } else if(password != confirmPass) {
        status.innerHTML = "<p class='red'>Password does not match!</p>";
    } else {
        _('signupBtn').disabled = true;
        status.innerHTML = "<p class='wait_msg'><i class='fa fa-cog fa-spin'></i> Please Wait...</p>";
        var ajax = ajaxObj("POST", "includes/signup_inc.php");
        ajax.onreadystatechange = function() {
            if(ajaxReturn(ajax) == true) {
                if(ajax.responseText != 'signup_success') {
                    status.innerHTML = ajax.responseText;
                    _('signupBtn').disabled = false;
                } else {
                    _('signupSuccess').innerHTML = "<div id='cardBox'><div class='card'><div class='card_front'><i class='fa fa-check' aria-hidden='true'></i></div><div class='card_back'>Success!</div></div></div><div id='successText'><p>Your account has been created successfully!</p><a href='login.php'>Click here to Log In</a></div>";
                }
            }
        }
        ajax.send("firstname="+firstname+"&lastname="+lastname+"&username="+username+"&email="+email+"&gender="+gender+"&phoneNum="+phoneNum+"&password="+password+"&confirmPass="+confirmPass);
    }
}

<?php
    if(isset($_POST['checkusername'])) {
        include 'dbconn.php';
        $username = mysqli_real_escape_string($conn, $_POST['checkusername']);
        $sql = "SELECT * FROM users WHERE username='$username'";
        $result = mysqli_query($conn, $sql);
        $usernameCount = mysqli_num_rows($result);
        if(strlen($username) < 4 || strlen($username) > 16) {
            echo "<p class='red'>4 - 16 Characters please!</p>";
            exit();
        } else if(is_numeric($username[0])) {
            echo "<p class='red'>Username must begin with letter!</p>";
            exit();
        } else if($usernameCount < 1) {
            echo "<p class='green'>$username is available!</p>";
            exit();
        } else {
            echo "<p class='red'>$username is already taken!</p>";
            exit();
        }
    }

    if(isset($_POST['username'])) {
        include 'dbconn.php';
        $firstname = mysqli_real_escape_string($conn, $_POST['firstname']);
        $lastname = mysqli_real_escape_string($conn, $_POST['lastname']);
        $username = mysqli_real_escape_string($conn, $_POST['username']);
        $email = mysqli_real_escape_string($conn, $_POST['email']);
        $gender = mysqli_real_escape_string($conn, $_POST['gender']);
        $phoneNum = mysqli_real_escape_string($conn, $_POST['phoneNum']);
        $password = mysqli_real_escape_string($conn, $_POST['password']);
        $confirmPass = mysqli_real_escape_string($conn, $_POST['confirmPass']);
        $ip = preg_replace('#[^0-9.]#', '', getenv('REMOTE_ADDR'));

        $sql = "SELECT * FROM users WHERE username='$username'";
        $result = mysqli_query($conn, $sql);
        $usernameCount = mysqli_num_rows($result);

        $sql = "SELECT * FROM users WHERE username='$username'";
        $result = mysqli_query($conn, $sql);
        $emailCount = mysqli_num_rows($result);

        if($firstname == "" || $lastname == "" || $username == "" || $email == "" || $gender == "" || $phoneNum == "" || $password == "" || $confirmPass == "") {
            echo "<p class='red'>Please fill out all fields!</p>";
            exit();
        } else if($password != $confirmPass) {
            echo "<p class='red'>Password does not match!</p>";
            exit();
        } else if($usernameCount > 0) {
            echo "<p class='red'>Username is already taken!</p>";
            exit();
        } else if($emailCount > 0) {
            echo "<p class='red'>Email address already exist!</p>";
            exit();
        } else if(is_numeric($username[0])) {
            echo "<p class='red'>Username must begin with letter!</p>";
            exit();
        } else if(strlen($username) < 4 || strlen($username) > 16) {
            echo "<p class='red'>4 - 16 Characters please!</p>";
            exit();
        } else if(!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            echo "<p class='red'>Invalid email address!</p>";
            exit();
        } else {
            $hashPass = password_hash($password, PASSWORD_DEFAULT);
            $capFirstname = ucwords($firstname);
            $capLastname = ucwords($lastname);

            $sql = "INSERT INTO users (firstname, lastname, username, email, gender, phone_number, password, ip)
            VALUES ('$capFirstname', '$capLastname', '$username', '$email', '$gender', '$phoneNum', '$hashPass', '$ip')";
            $result = mysqli_query($conn, $sql);
            echo "signup_success";
            exit();
        }
    }