PHP登录/注册系统错误

我在做一个登录/注册系统，参考了这个视频教程:PHP OOP登录/注册系统:数据库查询(第8/23部分):

我得到一个错误:

致命错误:在非对象上调用成员函数count()C:'wamp'www'login-register-systemv2'index.php on line 6

index . php

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

DB.php

这里是我的DB.php我认为问题是:

<?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()) {

        var_dump(count($where));
        if(count($where) === 3) {
            $operators = array('=', '>', '<', '>=', '<=');
            $field      = $where[0];
            $operator   = $where[1];
            $value      = $where[2];
            if(in_array($operator, $operators)) {
                $sql = "{$action} FROM {$table} WHERE {$field} {$operator} {$value} ?";
                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 count() {
        return $this->_count;
    }
    public function error() {
        return $this->_error;
    }
}

我不知道哪里不对，也许你可以。