PHP OOP登录/注册系统

我是通过观看youtube（phpacademy）上的教程开始构建oop登录/注册系统的，但我遇到了一个问题，代码不起作用。我查了几次，还是一无所获。数据库已经创建并运行良好，我在数据库（users）中有一个用户-亚历克斯。如果有人能看到我做错了什么，请帮助我：）

我希望我正确解释了我的问题，我得到的错误是："致命错误：在C:''examplep''htdocs''oopLogin''index.php的第7行对非对象调用成员函数count（）"

DB.php

<?php
class DB {
    private static $_instance = null;
    private $_pdo,
            $_query, 
            $_error = false,
            $_results,
            $_count = 0;
    private function __construct() {
        try {
            $this->_pdo = new PDO('mysql:host=' . Config::get('mysql/host') . ';dbname=' . Config::get('mysql/db'), Config::get('mysql/username'), Config::get('mysql/password'));          
        } catch(PDOException $e) {
            die($e->getMessage());
        }
    }
    public static function getInstance() {
        if(!isset(self::$_instance)) {
            self::$_instance = new DB();
        }
        return self::$_instance;
    }
    public function query($sql, $params = array()) {
        $this->_error = false;
        if($this->_query = $this->_pdo->prepare($sql)) {
            $x = 1;
            if(count($params)) {
                foreach($params as $param) {
                    $this->_query->bindValue($x, $param);
                    $x++;
                }
            }
            if($this->_query->execute()) {
                $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
                $this->_count = $this->_query->rowCount();
            }
            else {
                $this->_error = true;
            }
        }
        return $this;
    }
    public function action($action, $table, $where = array()) {
        if(count($where) === 3){
            $operators = array('=', '>', '<', '>=', '<=');
            $field    = $where[0];
            $operator = $where[1];
            $value    = $where[2];
            if(in_array($operator, $operators)) {
                $sql = "($action) FROM ($table) WHERE ($field) ($operator) ?";
                if(!$this->query($sql, array($value))->error()) {
                    return $this;
                }
            }
        }
        return false;
    }
    public function get($table, $where) {
        return $this->action('SELECT *', $table, $where);
    }
    public function delete($table, $where) {
        return $this->action('DELETE', $table, $where);
    }
    public function results() {
        return $this->_results;
    }
    public function error() {
        return $this->_error;
    }
    public function count() {
        return $this->_count;
    }
}

index.php

<?php
require_once 'core/init.php';
$user = DB::getInstance()->get('users', array('username', '=', 'alex'));
if(!$user->count()) {
    echo 'No user';
} else {
    foreach($user->results() as $user) {
        echo $user->username, '<br>';
    }
}