php - Mysqli->insert_id not returning anything


I'm creating a login/sign up page with PHP and MySQL, everything works just fine, but when I try to retrieve the insert_id from MySQL just doesn't work. Even if I try to "print" the result, it only says 1. Heres my code:

Here's my connection:

class Connection { //Connect to database
    /* Arguments */
    private $_host = "localhost";
    private $_username = ""; /* Change */
    private $_password = ""; /* Change */
    private $_database = "";
    private $_connection;
    private static $_instance;

    /* Methods */
    private function __construct(){ //Initial method
        $this->_connection=new mysqli($this->_host,$this->_username,$this->_password,$this->_database);
           trigger_error("Error al conectar a base de datos: " . $this->_connection->connect_error);
    private function __clone(){} //Magic method
    public static function getInstance(){ //Get instance of connection
            self::$_instance=new self();
        return self::$_instance;
    public function getConnection(){ //Get MySQLi connection
        return $this->_connection;

class Query extends Connection { //Query database
    public static function run($sql){
        return parent::getInstance()->getConnection()->query($sql);

And here's my users class:

class Users {
    /* Arguments */
    public static $result=NULL;

    /* Methods */
    public static function getDuplicated($sql){ //Get duplicated records

    public static function createUser($sql){ //Create new user on database

And finally this is where I call the INSERT:

$c=Users::createUser("INSERT INTO users (user,password,lastLogin) VALUES ('$username_register','$password_register',NOW())");
echo Users::$result;

As I said, everything works just fine, I can get records from database, it inserts correctly and so on, but when I user insert-id, it just gives nothing.

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