<?php
ini_set('display_errors',1); // enable php error display for easy trouble shooting
error_reporting(E_ALL); // set error display to all
session_start();

require_once "../../config/db.php";

if(isset($_POST)) {

    // you should validate all inputs and make sure you are getting what you are expecting
    // first you need to check empty
    // second you should use strip_tags for all inputs to avoid xss injection
    // Example: bad user may insert javascript to first name or last name and when you fetch the name, javascript will be loaded
    // Use FILTER_VALIDATE_EMAIL to validate email address http://php.net/manual/en/filter.filters.validate.php
    // check if email exists. If exists echo 'Error' with exit
    // use password_hash instead of MD5 for password http://php.net/manual/en/function.password-hash.php
    // use prepared statement to prevent sql injection https://stackoverflow.com/questions/60174/how-can-i-prevent-sql-injection-in-php?rq=1

    // to temporary fix your problem

    $check_empty = true;

    foreach ($_POST as $post) {
        if (empty($post)) {
            // value is empty
            $check_empty = false;
            echo "error";
            exit;
        }
    }

    $first_name = mysqli_real_escape_string($conn, $_POST['first_name']);
    $last_name = mysqli_real_escape_string($conn, $_POST['last_name']);
    $email = mysqli_real_escape_string($conn, $_POST['email']);
    $password = mysqli_real_escape_string($conn, $_POST['password']);
    $user_birthday = mysqli_real_escape_string($conn, $_POST['user_birthday']);
    $user_sex = mysqli_real_escape_string($conn, $_POST['user_sex']);

    $password = base64_encode(strrev(md5($password)));
    $dob = date('Y-m-d',strtotime($user_birthday));
    if ($check_empty) {
        $sql = "SELECT email FROM users WHERE email = '$email' LIMIT 1";
        if ($result = $conn->query($sql)) {
            if ($result->num_rows > 0) {
                echo "error";
                exit;
            }
        } else {
            $sql = "INSERT INTO users(first_name, last_name, email, password, user_birthday, user_sex) VALUES ('$first_name', '$last_name', '$email', '$password', '$dob', '$user_sex')";

            if($conn->query($sql)===TRUE) {
                $_SESSION['registeredSuccessfully'] = true;
                echo "ok";
                exit;
            } else {
                echo "error";
                exit;
            }
        }
    }

}

?>